differernt types of Operon concepts

total description of lactose operon, galactose operon, arabinose operon, tryptophan operon and their regulations and mechanisms

1. Operon concept s D.INDRAJA

2. The basic concept about how gene regulation occurs at the level of transcription in bacteria was provided by the classical model called operon model(formulated by jacob and monad in 1961) Operons consist of multiple genes grouped together with a promoter and an operator. Operons are present in prokaryotes ( bacteria  and archaea ), but are absent in eukaryotes. Operon Structure Operons are regions of DNA that contain clusters of related genes. They are made up of a promoter region, an operator, and multiple related genes. The operator can be located either within the promoter or between the promoter and the genes.   RNA polymerase  initiates  transcription  by binding to the promoter region. The location of the operator is important as its regulation either allows or prevents transcription of the genes into  mRNA .

3. The product of lac operon is involved in the conversion of the disachharide sugar lactose in to its monosachharide units glucose and galactose Operons can be under negative or positive control. Negative control involves turning off the operon in the presence of a repressor molecule . The repressor binds to the operator in such a way that the movement or binding of RNA polymerase is blocked and transcription cannot proceed.

4. An inducible operon is one that is usually off. In the absence of an inducer the operator is blocked by a repressor molecule. When the inducer is present it interacts with the repressor protein, releasing it from the operator and allowing transcription to proceed.

5. What makes the  lac   operon turn on? E. coli  bacteria can break down lactose, but it's not their favorite fuel. If glucose is around, they would much rather use that. Glucose requires fewer steps and less energy to break down than lactose. However, if lactose is the only sugar available, the  E. coli  will go right ahead and use it as an energy source. To use lactose, the bacteria must express the  lac   operon genes, which encode key enzymes for lactose uptake and metabolism. To be as efficient as possible,  E. coli  should express the  lac   operon only when two conditions are met: Lactose is available,  and Glucose is  not  available How are levels of lactose and glucose detected, and how how do changes in levels affect  lac   operon transcription? Two regulatory proteins are involved: One, the  lac  repressor, acts as a lactose sensor. The other, catabolite activator protein (CAP), acts as a glucose sensor. These proteins bind to the DNA of the  lac   operon and regulate its transcription based on lactose and glucose levels.

6. Structure of the  lac   operon The  lac   operon contains three genes:  lacZ ,  lacY , and  lacA . These genes are transcribed as a single mRNA, under control of one promoter. Galactoside permease  (also known as lactose permease ).( lac Y)   Transports lactose into the cell across the cell membrane . Galactosidase ( lac Z)  Hydrolyzes lactose to glucose and galactose . Thiogalactoside transacetylase ( lac A)

7. In addition to the three genes, the  lac   operon also contains a number of regulatory DNA sequences. These are regions of DNA to which particular regulatory proteins can bind, controlling transcription of the operon . The  promoter  is the binding site for RNA polymerase, the enzyme that performs transcription. The  operator  is a negative regulatory site bound by the  lac  repressor protein. The operator overlaps with the promoter, and when the  lac  repressor is bound, RNA polymerase cannot bind to the promoter and start transcription. The  CAP binding site  is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter. The  lac  repressor The  lac  repressor is a protein that represses (inhibits) transcription of the  lac   operon . It does this by binding to the operator, which partially overlaps with the promoter. When bound, the  lac  repressor gets in RNA polymerase's way and keeps it from transcribing the operon .

8. The gene that encodes the lac repressor is named  lacI , and is under control of its own promoter. The  lacI  gene happens to be found near the  lac   operon , but it is not a part of the operon and is expressed separately.  lacI  is continually transcribed, so its protein product – the  lac  repressor – is always present. When lactose is not available, the  lac  repressor binds tightly to the operator, preventing transcription by RNA polymerase. However, when lactose is present, the  lac  repressor loses its ability to bind DNA. It floats off the operator, clearing the way for RNA polymerase to transcribe the operon .

9. The  lac   inducer This change in the  lac  repressor is caused by the small molecule called inducer which is a allolactose , an isomer (rearranged version) of lactose. When lactose is available, some molecules will be converted to allolactose inside the cell. Allolactose binds to the  lac  repressor and makes it change shape so it can no longer bind DNA. Allolactose is an example of an  inducer , a small molecule that triggers expression of a gene or operon . The  lac   operon is considered an  inducible operon  because it is usually turned off (repressed), but can be turned on in the presence of the inducer allolactose . Another inducer of the lac operon is isopropylthiogalactoside (IPTG). Unlike allolactose , this inducer is not metabolized by  E. coli  and so, is useful for experimental studies of induction only. Lac Operon in absence of Inducers In the absence of an inducer such as allolactose or IPTG, the lacI gene is transcribed and the resulting repressor protein binds to the operator site of the lac operon , Olac , and prevents transcription of the lacZ , lacY and lacA genes .

10. Catabolite activator protein (CAP) RNA polymerase alone does not bind very well to the  lac   operon promoter. It might make a few transcripts, but it won't do much more unless it gets extra help from  catabolite activator protein  ( CAP ). CAP binds to a region of DNA just before the  lac   operon promoter and helps RNA polymerase attach to the promoter, driving high levels of transcription . CAP isn't always active (able to bind DNA). Instead, it's regulated by a small molecule called  cyclic AMP  ( cAMP ). cAMP is a "hunger signal" made by  E. coli  when glucose levels are low. cAMP binds to CAP, changing its shape and making it able to bind DNA and promote transcription. Without cAMP , CAP cannot bind DNA and is inactive. CAP is only active when glucose levels are low ( cAMP levels are high). Thus, the  lac   operon can only be transcribed at high levels when glucose is absent. This strategy ensures that bacteria only turn on the  lac   operon and start using lactose after they have used up all of the preferred energy source (glucose).

11. Lac operon in the presence of Glucose When glucose is present ,  E. coli  does not need to use lactose as a carbon source and so the lac operon does not need to be active. Thus the system has evolved to be responsive to glucose. Glucose inhibits adenylate cyclase , the enzyme that synthesizes cAMP from ATP. Thus, in the presence of glucose the intracellular level of cAMP falls, so CRP cannot bind to the lac promoter, and the lac operon is only weakly active (even in the presence of lactose). Lac operon in the absence of Glucose When glucose is absent , adenylate cyclase is not inhibited, the level of intracellular cAMP rises and binds to CRP. Therefore, when glucose is absent but lactose is present, the CRP– cAMP complex stimulates transcription of the lac operon and allows the lactose to be used as an alternative carbon source. In the absence of lactose, the lac repressor, of course, ensures that the lac operon remains inactive. These combined controls ensure that the lacZ , lacY and lacA genes are transcribed strongly only if glucose is absent and lactose is present .

12. Negative Control of the  lac   Operon The protein that inhibits transcription of the  lac   operon is a tetramer with four identical subunits called  lac  repressor. The  lac  repressor is encoded by the  lacI  gene, located upstream of the  lac   operon and has its own promoter. Expression of the  lacI  gene is not regulated and very low levels of the  lac  repressor are continuously synthesized. Genes whose expression is not regulated are called constitutive genes. In the absence of lactose the  lac  repressor blocks the expression of the  lac   operon by binding to the DNA at a site, called the operator that is downstream of the promoter and upstream of the transcriptional initiation site. The operator consists of a specific nucleotide sequence that is recognized by the repressor which binds very tightly, physically blocking (strangling) the initiation of transcription.

13. Positive and Negative Regulation of Lac Operon The lac operon is a good example of the negative control (negative regulation) of gene expression in that bound repressor prevents transcription of the structural genes. Positive control or regulation of gene expression is when the regulatory protein binds to DNA and increases the rate of transcription. In this case, the regulatory protein is called an activator. The CAP/CRP involved in regulating the lac operon is a good example of an activator. Thus the lac operon is subject to both negative and positive control. Negative regulation

14. positive regulation Glucose CAP binds Lactose Repressor binds Transcription + – – + No + – + – Some – + – + No – + + – Yes

15. Diauxic growth curve A  diauxic growth curve  refers to the  growth curve  generated by an  organism  which has two growth peaks. The theory behind the diauxic growth curve stems from  Jacques Monod's  Ph.D. research in 1940. A simple example involves the  bacterium   Escherichia coli  ( E. coli ), the best understood bacterium. The bacterium is grown on a  growth media  containing two types of  sugars , one of which is easier to  metabolize  than the other (for example  glucose  and  lactose ).

16. First, the bacterium will metabolize all the glucose, and grow at a higher speed. Eventually, when all the glucose has been consumed, the bacterium will begin the process of expressing the  genes  to metabolize the lactose. This will only occur when all glucose in the media has been consumed. For these reasons,  diauxic growth  occurs in multiple phases.

17. Structure of galactose operon Out side the operon (far from transcription site) GalR is present which synthesizes repressor molecule for the galactose operon The gal operon contains two  operators ,  O E  (for external) at -60.5 postion and  O I  (for internal) at +53.5 position. These operators bind the repressor,  GalR ,  which is encoded from outside the operator region. For this repressor protein to function properly, the operon also contains a histone binding site( at 6.5 position) to facilitate this process. it has two overlapping promoters P1(upper phase) &P2 (lower phase) are present having transcription sites at +1 &-5 positions

18. An additional site, known as the activating site, is found following the external operator, but upstream of P2. This site serves as the binding region for the cAMP-CRP complex, which modulates the activity of the promoters and thus, gene expression. The galactose operon has 4 structural genes galE  UDP galactose 4-epimerase galT  galactose -1- phosphate uridylyl transferase galK  galactokinase galM  mutarotase A part from these genes galR  it is a unlinked gene that encodes repressor for the gal operon How does the galctose form in the cell? Galactose may enter in to the cell by permeases β -D-galactose generated from lac operon enzyme β -galactosidase( when lac operon is on the enzyme galactosidase breaks in to galctose and galactosidase)

19. It links the enzymes of lactose and galactose metabolism in to common pathway GALACTOSE METABOLISM

20. Gal R mediated DNA loop formation/repression

21. The binding of GalR to OE represses P1 and activates P2 GalR bound to OE is located on the same DNA face as RNAP bound to P1 , but on the opposite DNA face as RNAP bound to P2 GalR represses P1 by inhibiting the rate determining open complex formation through RNAP contacts . This mode of repression is termed “contact inhibition” While P1 is repressed by GalR -OE complex, P2 is activated by GalR -OE complex by a direct contact between GalR and RNAP, “contact activation” . GalR -OE enhances open complex formation at P2 where inhibits open complex formation in P1 so P1 is repressed and P2 is enhanced

22. 4 . Regulation of gal promoters by CAMP CRP complex (CCC) Role of CCC The gal operon includes a 16-bp activating site (AS) located at 40.5 that binds the regulator CCC for activating P1 and repressing P2 The overlapping of the AS at 40.5 with the 35 element of P1 is a feature of CCC-regulated Class II promoters. In contrast, in Class I promoters, the AS is located upstream (61.5) to the promoter region for RNA polymerase (RNAP). CCC represses P2 by decreasing open complex formation of RNAP. In contrast, CCC activates P1 by increasing both closed complex formation and isomerization from the closed complex to the open complex of RNAP . The AS of CCC is located on the same face as RNAP at P1 and on the opposite face of RNAP at P2. By binding to AS, CCC switches transcription initiation from P2 to P1. Glucose high  CRP CAMP levels Glucose low  CRP CAMP levels (Only when lactose or galactose are present in the medium)

23. Gal Isorepressor The gal operon is also regulated by an isorepressor ( GalS ). Although GalS does not seem to repress the gal promoters by DNA looping, the isorepressor does stimulate P2 and repress P1, the same way that GalR does by binding to OE, except that the effects are weaker.  GalR and GalS are 85% similar in their amino acid sequences .  By their homology with proteins ( GalR-LacI family) whose structures are known

24. Introduction The 20 common amino acids are required in large amount for protein synthesis, and E.coli can synthesize all of them. The genes for the enzymes needed to synthesize a given amino acid are generally clustered in an operon and are expressed whenever existing supplies of that amino acid are inadequate for cellular requirements. When the amino acid is abundant the biosynthetic enzymes are not needed and the operon is repressed Bacteria such as  Escherichia coli   (a friendly inhabitant of our gut) need amino acids to survive—because, like us, they need to build proteins. One of the amino acids they need is tryptophan . If tryptophan is available in the environment,  E. coli  will take it up and use it to build proteins. However,  E. coli  can also make their own tryptophan using enzymes that are encoded by five genes. These five genes are located next to each other in what is called the  trp  operon . If tryptophan is present in the environment , then  E. coli  bacteria don't need to synthesize it, so transcription of the genes in the   trp  operon is switched "off." When tryptophan availability is low , on the other hand, the operon is switched "on," the genes are transcribed, biosynthetic enzymes are made, and more tryptophan is produced.

25. Structure of the  trp  operon The  trp  operon includes five genes that encode enzymes needed for tryptophan biosynthesis, along with a  promoter  (RNA polymerase binding site) and an  operator  (binding site for a repressor protein). The genes of the  trp  operon are transcribed as a single mRNA. The trp operator region partly overlaps the trp promoter. The operon is regulated such that transcription occurs when tryptophan in the cell is in short supply

26. Structural genes are TrpE , TrpD , TrpC , TrpB and TrpA trpE : It enodes the enzyme Anthranilate synthase I trpD : It encodes the enzyme Anthranilate synthase II trpC : It encodes the enzyme N-5’-Phosphoribosyl anthranilate isomerase and Indole-3-glycerolphosphate synthase trpB : It encodes the enzyme tryptophan synthase-B sub unit trpA : It encode the enzyme tryptophan synthase-A sub unit trpE trpD trpC trpB / trpA trpC

27. Mechanisms of operon Repression Attenuation Repression Turning the operon "on" and "off“ operator stretch of DNA is recognized by a regulatory protein known as the  trp  repressor . When the repressor binds to the DNA of the operator, it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme. trp  repressor The  trp  repressor protein is encoded by a gene called  trpR . This gene is not part of the  trp  operon, and it's located elsewhere on the bacterial chromosome, where it has its own promoter and other regulatory sequences . Translation mediated RNA binding protein mediated

28. The  trp  repressor does not always bind to DNA. Instead, it binds and blocks transcription only when tryptophan is present. When tryptophan is around, it attaches to the repressor molecules and changes their shape so they become active. A small molecule like trytophan , which switches a repressor into its active state, is called a  corepressor . COREPRESSOR

29. In the Presence of Tryptophan When tryptophan is present, the enzymes for tryptophan biosynthesis are not needed and so expression of these genes is turned off. This is achieved by tryptophan binding to the repressor to activate it so that it now binds to the operator and stops transcription of the structural genes. Binding of repressor protein to operator overlaps the promoter, so RNA polymerase cannot bind to the prometer . Hence transcription is halted. In this role, tryptophan is said to be a co-repressor. This is negative control, because the bound repressor prevents transcription.

30. In the Absence of Tryptophan In the absence of tryptophan, a trp repressor protein encoded by a separate operon, trpR , is synthesized and forms a dimer. However, this is inactive and so is unable to bind to the trp operator and the structural genes of the trp operon are transcribed.

31. Attenuation Translation mediated A second mechanism, called attenuation, is also used to control expression of the trp operon. The 5′ end of the polycistronic mRNA transcribed from the trp operon has a leader sequence upstream of the coding region of the trpE structural gene. This section lies between the operator and the first gene of the operon and is called the  leader . The leader encodes a short polypeptide and also contains an  attenuator  sequence. The attenuator does not encode a polypeptide, but when transcribed into mRNA, it has self-complementary sections and can form various hairpin structures. This  leader sequence  encodes a 14 amino acid leader peptide containing two tryptophan residues. The function of the leader sequence is to fine tune expression of the trp operon based on the availability of tryptophan inside the cell.

32. The leader sequence contains four regions (numbered 1–4) that can form a variety of base paired stem-loop (‘hairpin’) secondary structures. The regions are: Region 1, region 2, region 3 and Region 4. Region 3 is complementary to both region 2 and region 4. If region 3 and region 4 base pair with each other, they form a loop like structure called attenuator and it function as transcriptional termination. If pairing occur between region 3 and region 2, then no such attenuator form so that transcription continues .

33. What mechanism determines whether sequence 3 pairs with sequence 2 or with sequence 4 ? When tryptophan concentrations are high, concentrations of charged tryptophan tRNA are also high. This allows translation to proceed rapidly past the two trp codons of sequence 1 and into sequence 2, before sequence 3 is synthesized by RNA polymerase. In this situation, sequence 2 is covered by the ribosome , and unavailable for paring to sequence 3 when sequence 3 is synthesized; the attenuator structure sequence 3 and 4 forms and transcription halts. When tryptophan concentrations are low, the ribosome stalls at the two Trp codons in sequence 1 because the charged tryptophan tRNA is unavailable. • Sequence 2 remains free while sequence 3 is synthesized, allowing these two sequences to base pair and permitting transcription to proceed.

34. Attenuation depends on the fact that, in bacteria, ribosomes attach to mRNA as it is being synthesized and so translation starts even before transcription of the whole mRNA is complete. When Trypophan is abundant If there is plenty of tryptophan, the ribosome won't have to wait long for a tryptophan-carrying tRNA, and will rapidly finish the leader polypeptide. If the ribosome translates quickly, it will fall off the mRNA after translating the leader peptide. This allows the terminator hairpin and an associated hairpin to form, making RNA polymerase detach and ending transcription. When tryptophan is abundant, ribosomes bind to the trp polycistronic mRNA that is being transcribed and begin to translate the leader sequence. Now, the two trp codons for the leader peptide lie within sequence 1, and the translational Stop codon lies between sequence 1 and 2. During translation, the ribosomes follow very closely behind the RNA polymerase and synthesize the leader peptide, with translation stopping eventually between sequences 1 and 2. At this point, the position of the ribosome prevents sequence 2 from interacting with sequence 3. Instead sequence 3 base pairs with sequence 4 to form a 3:4 stem loop which acts as a transcription terminator. Therefore, when tryptophan is present, further transcription of the trp operon is prevented.

35. When Tryptophan is scarce If there is little tryptophan, the ribosome will stall at the Trp codons (waiting for a Trp -carrying tRNA) and will be slow to finish translation of the leader. If the ribosome translates slowly, it will pause, and its pausing causes formation of the antiterminator (non-terminating hairpin). This hairpin prevents formation of the terminator and allows transcription to continue. If, however, tryptophan is in short supply, the ribosome will pause at the two trp codons contained within sequence 1. This leaves sequence 2 free to base pair with sequence 3 to form a 2:3 structure (also called the anti-terminator), so the 3:4 structure cannot form and transcription continues to the end of the trp operon. Hence the availability of tryptophan controls whether transcription of this operon will stop early (attenuation) or continue to synthesize a complete polycistronic mRNA.

36. Conclusion This mechanism may be complex, but the result is pretty straightforward. When tryptophan is abundant  the ribosome moves quickly along the leader, the terminator hairpin forms, and transcription of the  trp  operon ends. When tryptophan is scarce  the ribosome moves slowly along the leader, the anti -terminator hairpin forms, and transcription of the  trp  operon continues. In other words, the logic of attenuation is the same as that of regulation by the  trp  repressor. In both cases, high levels of tryptophan in the cell shut down the expression of the operon. This makes sense, since high levels of tryptophan mean that the cell does not need to make more biosynthetic enzymes to produce additional tryptophan . RNA binding protein mediated In B.subtilis transcriptional attenuation of trp operon is mediated by RNA binding terminator protein called TRAP( Trp -binding A ttenuation Protein) TRAP forms a multimer of 11 subunits and each subunit binds a single tryptophan amino acid .TRAP is activated by binding to the amino acid tryptophan TRAP bind to a sequence in the leader segment of transcript. Binding of TRAP causes the formation of termination hairpin. The termination of transcription then prevents the production of the tryptophan biosynthetic enzymes

37. In the absence of tryptophan, TRAP doesnot bind and the mRNA adopts a structure that prevents the terminator hairpin from forming. Protein anti-TRAP controls the activity of TRAP, which binds to TRAP, and prevents it from repressing the tryptophan operon

38. ARABINOSE OPERON

39. The  L-arabinose operon , also called the  ara  or  araBAD  operon , is an  operon  required for the breakdown of the five-carbon sugar  L-arabinose  in  Escherichia coli . Arabinose is another non-glucose carbohydrate source, for which bacterial cells have genetic capabilities for utilizing it as an alternative source of carbon. For utilization of Arabinose several genes operate. Few genes are required for its transport into the cell.  Another set of gene products is required for the conversion of Arabinose into Xylulose.  Then another set of enzymes is required for converting Xylulose into Fructose1,6-di phosphate, which becomes the part of glycolytic  pathway. (pentose phosphate pathway) Multiple gene metabolic pathway is regulated by a common protein complex; so it is called ‘ Arabinose Regulon’ . It is a Complex regulatory system First studied by Ellis Englesberg Arabinose operon can be regulated both positively and negatively Exhibits catabolite repression

40. Structure L-arabinose operon is composed of structural genes and regulatory regions including the operator region ( araO 1 ,  araO 2 ) and the inducer region ( araI 1 ,  araI 2 ).  The structural genes,  araB ,  araA  and  araD , encode enzymes for L-arabinose  catabolism . There is also a CAP binding site where CAP-cAMP complex binds to and facilitates  catabolite repression , and results in positive regulation of  ara BAD  when the cell is starved of  glucose

41. The regulatory gene,  araC , is located upstream of the L-arabinose operon and encodes the arabinose-responsive  regulatory protein   AraC . Both  ara C  and  ara BAD  have a discrete promoter where  RNA polymerase  binds and initiates  transcription . araBAD  and  araC  are transcribed in opposite directions from the  araBAD  promoter ( P BAD ) and  araC  promoter ( P C ) respectively Structural genes araA    L-arabinose isomerase which catalyses  isomerization between  L-arabinose and  L-ribulose araB      ribulokinase which catalyses  phosphorylation  of L-ribulose to form L- ribulose-5-phosphate . araD    L-ribulose-5-phosphate 4-epimerase , which catalyses  epimerization  between L-ribulose 5-phosphate and D- xylulose-5-phosphate

42. Regulation The araC gene is adjacent to the ara operon It has its own promoter, It encodes a regulatory protein, AraC AraC can bind to three different operator sites ,Designated araI , araO1 and araO2  When araC protein is depleted, the ara C gene is transcribed from its own promoter The AraC protein can act as either a negative or positive regulator of transcription Depending on whether or not arabinose is present

43. AraC functions as a  homodimer , which can control transcription of  araBAD  through interaction with the operator and the initiator region on L-arabinose operon. Each AraC   monomer  is composed of two domains including a  DNA binding domain  and a  dimerisation  domain. The dimerisation domain is responsible for arabinose-binding.   AraC undergoes  conformational change  upon arabinose-binding, in which, it has two distinct conformations.The conformation is purely determined by the binding of  allosteric   inducer  arabinose

44. Negative regulation of  araBAD When arabinose is absent, cells do not need the  ara BAD  products for breaking down arabinose. Therefore, dimeric AraC acts as a repressor: one monomer binds to the operator of the  araBAD  gene ( araO 2 ), another monomer binds to a distant  DNA  half site known as  araI .  This leads to the formation of a DNA loop. This orientation blocks RNA polymerase from binding to the  araBAD  promoter. Therefore, transcription of structural gene  araBAD  is inhibited.

45. Positive regulation of  araBAD Expression of the  araBAD  operon is activated in the absence of glucose and in the presence of arabinose. When arabinose is present, both AraC and CAP work together and function as activators. Via AraC AraC acts as an activator in the presence of arabinose. AraC undergoes a conformational change when arabinose binds to the dimerization domain of AraC . As a result, the AraC -arabinose complex falls off from  araO 2  and breaks the DNA loop. Hence, it is more  energetically  favourable for AraC -arabinose to bind to two adjacent DNA half sites:  araI 1  and  araI 2  in the presence of arabinose. One of the monomers binds  araI 1 , the remaining monomer binds  araI 2  - in other words, binding of AraC to  araI 2  is  allosterically  induced by arabinose. One of the AraC monomers places near to the  araBAD  promoter in this configuration, which helps to recruit RNA polymerase to the promoter to initiate transcription

46. CAP/ cAMp CAP act as a transcriptional activator only in the absence of  E. coli' s preferred sugar, glucose. When  glucose  is absent, high level of CAP protein/cAMP complex bind to CAP binding site, a site between  araI 1  and  araO 1 . Binding of CAP/cAMP is responsible for opening up the DNA loop between  araI 1  and  araO 2 , increasing the binding affinity of AraC protein for  araI 2  and thereby promoting RNA polymerase to bind to  araBAD  promoter to switch on the expression of the  araBAD  required for metabolising L-arabinose .

47. Autoregulation of AraC The expression of  araC  is negatively regulated by its own protein product, AraC . The excess AraC binds to the operator of the  araC  gene,  araO 1 , at high AraC levels, which physically blocks the RNA polymerase from accessing the  araC  promoter. Therefore, the AraC protein inhibits its own expression at high concentrations

48. Mutations in the arabinose operon   A mutation in the araA gene will cause the bacterial cell to become arabinose negative. Bacteria no longer uses arabinose as carbon source. araB gene mutation results the same. araD gene mutation results in cell death. Mutation of araC gene causes the promoters ( pBAD ) and pC to become inactive, permanently repressed. Similarities between lactose- and arabinose-operon: Both of them contains three structural genes. • Inducible When glucose is present at high levels it prevents both the synthesis of arabinose and lactose. The function of both the operons are parallel. they undergo positive and negative regulation The products formed in both the operons are similar, positive regulation : sugars are formed negative regulation : no sugar