Structure,genome organisation,gene regulation of lambda phage

description of structure and discovery and genome organization and the lytic and lysogenic cycle and gene regulation and replication of lambda phage

1. STRUCTURE,GENOME ORGANISATION,GENE REGULATION OF LAMBDA PHAG E D.INDRAJA

2. It has a j chain instead of basal plate and tail fibres as in the case of t4 phage.and the electron microscopic image of lambda page shows it has a flexible tail where as t4 phage has contractile tail .and it is also called as a temperate phage because as it undergoes lysogenic cycle

3. During the discovery they had observed that a particular e.coli which acts as a donor when it undergoes conjugation it results in the lysis of the recipient cell. And in the other case when the same donor e.coli subjected to u.v radiation then it also results in the cell lysis by forming plaques. But donor e.coli it doesn’t undergo lysis so they concluded that there is a factor that is present in the dormant form in the donor e.coli that transfers in to recipient or activates under the uv radiation that results in the lysis of the cell (which is non other than prophage ).

4. In general they are present in the linear form in the lambda phage but when the lambda phage infects the bacteria then they get circularized due to the presence of cohesive ends because if not circularized they may get degraded with exonucleases present in the host bacteria

5. LYTIC AND LYSOGENIC CYCLE

6. GENE REGULATION-CHOICE BETWEEN LYTIC AND LYSOGENIC CYCLE They are two different types of gene cro gene and N gene both has separate operator promoter and terminator cro gene on the right and N gene on the left when RNA polymerase bind to the promoter (R) then the transcription occurs on the right side and when the RNA polymerase binds to the promoter (L) the the transcription occurs on the left side. When virus enters enters in to the bacteria the RNA polymerase of the bacteria binds to the promoter (L) and then transcribing the N gene and protein products of N gene have been appeared these act as a anti terminator proteins for the regions (tL1) and (tR2).similarly when the RNA polymerase binds to the promoter (R) then cro gene has been transcribed and protein products of cro gene have been synthesized and it stops at the terminator (tR1) in 50% time(Fig-2).and in the other case RNA polymerase doesnot end the terminator(tR1) and further transcribes then it results in the synthesis of some other gene products such as CII ,O,P and these three genes have a terminator region of (tR2) in (Fig-3) as the N gene products act as anti terminatorproteins which have been already synthezied does not block the transcription on both the sides of right and left .in the left side it further transcribes a gene products called ( int ) integrase which acts a integration of viral genome in to and out of the bacterial genome.and ( Xis ) which is useful for excision which plays a major role during the replicationof viral genome and CIII gene product at this time N gene products have been synthezied in needed amount. Fig-1

7. On the other hand in the right side synthesis of Q gene product have been done and these also act as a anti terminator proteins and in this time there synthesizes more amount of cro proteins and CII,and O,P proteins as the more amount of cro proteins are present they undergo bind with the operator regions of both right and left and stops the synthesizing of the all proteins that have been mentioned before(Fig-4) .now the Q protein which act as a anti terminator protien that undergoes and gives middle genes and further late genes which are useful for synthesizing the new virus particles and produce the enzyme which is useful in the dissolving the cell wall of bacteria and lysis of the cell and release of viral particles have been occurred(Fig-5) Fig-2

8. Fig-3 Fig-4

9. Fig-5 In general the lambda phage genome undergoes two types of replication they are theta mode of replication and rolling circle mode of replication When the viral genome enters in the bacteria it undergoes theta mode of replication because there is a protein called Rec BCD which inhibits the rolling circle mode of replication . when the bacteria undergoes lytic cycle it synthesizes a protein called Gam w hich inhibits the Rec BCD protein ( i,e inhibiting the theta mode of replication) so the bacteria then undergoes lytic cycle (Fig-6)

10. Fig-6 During the replication of viral genome it replicates linearly that containing cos sites and after the replication with the help of an gene product( Xin ) which helps in excision of cos sites and further packages the genome in to individual viral particles

11. And in the other case there is a gene called CI and has other promoters called P RE and PI all these are present on the other strand of DNA(the cro genes and N genes are present on one strand of DNA and CI gene is present on the other strand ) and the proteins CII and CIII are present in the cell CII are unstable they can be digested with the host enzymes but they get stabilized by binding with CIII and making complex of (CII-CIIII) proteins these complex proteins bind to both the regions of P RE and PI and RNA polymerase comes and start the transcription of CI gene products(lambda repressor) and this also produces integrase as they also binds with (PI) promoter. Now there is a needed amount of CI gene products the undergo bind with the P RM gene and also bind with the operator right and left(OR,OL) here the CI gene products as both activator and repressor (activator when binds with P RM i,e for its own gene and repressor when it binds with the regions of OR and OL and stops the transcription of the other genes). So that it can not enter in to the lytic cycle now with the help of integrase it enters in to the bacterial genome and forms prophage (Fig-7)

12. Fig-7 How the lytic cycle is stopped during the lysogenic cycle In the OR( perator right) region there are three regions OR1,OR2,OR3 (0R1 is present towards the cro gene and OR3 is present towards the CI gene) (Fig-8)the CI gene product has high affinity to binding to ORI (OR1>OR2>OR3) so when the CI binds to OR1 RNA polymerase can not recognises the promoter region of cro gene so it stops the transcription and futher binding to OR2 and OR3 and if more the CI genes are present it also get block the production of CI gene(Fig-9)

13. Fig-8

14. Fig-9

15. During the prophage formation there is a site called attp (attachment phage) similarly there is another site in the bacterial genome called attb (attachment bacteria) with the help of integrase and ligase the lambda genome and bacterial genome integrates by homologous recombination forming lambda prophage and the integrase enzyme also have the other property of excision from the prophage (Fig-10) Fig-10

16. There is a protein called Rec A in the bacteria when it gets activated such as uv induction it undergoes clevage of CI gene(lambda repressor) which is present in the form of dimer gets converted in to monomer so that it is cleaved of from the operator sites and finally the bacteria undergoes lytic cycle(Fig-11) Fig-11

17. SUMMARY Cro –repressor CI - repressor and activator -activates CI gene transcription