Plasmid: types, structure and functions.

Plasmid: types, structure and functions.

1. PLASMIDS Submitted to Submitted by Dr. Aghil soorya Krishnapriya M D epartment of Botany Roll no: 10 St.Teresas college 1 st Msc Botany

2. JOSHUA LEDERBERG The term ‘ plasmid’ was first introduced by the American molecular Biologist Joshua Lederberg In 1952. Known for his work in Genetics Artificial intelligence Space exploration Won Nobel prize in Physiology or Medicine for discovering that bacteria can mate and exchange genes.

3. WHAT IS PLASMID Plasmids are circular, double-stranded DNA molecules that exist independently of host chromosomes. Present in many bacteria and also found in yeasts. Have their own replication origins and are autonomously replicating and stably inherited. A replicon is a DNA molecule or sequence that has a replication origin and is capable of being replicated. Plasmids and bacterial chromosomes are separate replicons. Plasmids have relatively few genes, generally less than 30.

4. Plasmid cont … Their genetic information is not essential to the host and bacteria that lack them usually function normally. They are extra chromosomal DNA molecule. Plasmid DNA may sometimes be integrated with chromosomal DNA of bacteria. Such integrated forms are called ‘ episomes ’. They can carry genes for toxins and proteins that promote transfer of plasmid to other bacterial cells. Size of plasmid varies from less than 1.0kb to more than 200kb. Smaller plasmids are much desirable for gene cloning experiments. Larger plasmids are less in number whereas smaller ones are more.

5. TYPES OF PLASMIDS Based on function: PLASMIDS CONJUGATIVE NON-CONJUGATIVE

6. Types of plasmid cont … CONJUGATIVE PLASMIDS Plasmids are transferred from bacterium to bacterium (usually members of same species or very closely related species) through conjugation. To this group belong R plasmid, F plasmid and certain bacteriocinogen plasmids. Conjugative plasmids are common in Gram-negative bacilli. They are relatively large-approximately 250kb in size. Large plasmids are usually present at 1-2 copies per bacterium. The replication of these plasmids are closely linked to that of bacterial chromosome.

7. Types of plasmid cont … NON-CONJUGATIVE PLASMIDS Common in Gram-positive bacteria. But also seen in some gram-positive organisms. Usually small, 1-10kb in size. Multiple copies may be present in each bacterium. They can also transferred to another cell when the same bacterium carries both conjugative and non-conjugative plasmids. Once conjugation is established, the donor can transfer non-conjugative plasmids.

8. CONJUGATION? It is a process by which a ‘male’ or ‘donor’ bacterium mates or make physical contact with a ‘female’ or ‘recipient’ bacterium and transfer genetic elements. In conjugation, genetic exchange between bacteria requires direct cell to cell contact during mating of bacteria. Bacterial conjugation in E. coli was discovered by Lederberg and Tatum in 1946, when they observed sex-like exchange between two mutant strains of E.coli called k12. Conjugation results in the one-way transfer of DNA from a donor cell to a recipient cell through sex pilus .

9. Conjugation cont … Only bacteria that contain F plasmid can act as donors, while bacteria lacking F act as recipients. Conjugation involves two cell types: donors as F+ male cells , which possess F factor and recipients are called F- cells as they lack F factor. Once F factor is passed from F+ cell to F- cell, the recipient cell becomes F+ and is able to pass fertility factor to another F- cell.

10. F-FACTOR The fertility factor possess genetic information required for synthesis of sex pilus and for self transfer. It do not contain other identifiable genetic markers such as drug resistance. When plasmid containing F factor becomes integrated into bacterial chromosome, cells are known as Hfr cells. Hfr cells are high frequency recombinant strains of cells because such cells are able to transfer chromosomal genes to recipient cells at high frequency. The mating between Hfr cells and F- cells results in transfer of part of F genome plus some host cell genes from donor cell.

11. F factor cont … The recipient F- cells usually remain F- after conjugation. Because only part of F plasmid from donor Hfr cell to recipient cell has been transferred by conjugative process. Thus recipient cell does not turn into Hfr and is unable to serve as a conjugation does.

12. STRUCTURE The most common functional segments constituting F factors are: OriT (Origin of Transfer): The sequence which marks starting point of conjugative transfer. OriV (Origin of Vegetative Replication): The sequence starting with which plasmid DNA will be replicated in the recipient cell. tra -region : Genes coding the F pilus and DNA transfer process. IS (Insertion Elements): Composed of one copy of IS2, two copies of IS3 and one copy of IS1000; so called ‘selfish genes’.

13. FUNCTIONS The F- factor allows donor to produce a thin, tubelike structure called a pilus , which the donor uses to contact the recipient. The pilus then draws the two bacteria together, at which donor bacterium transfer genetic material to the recipient. It control sexual functions of bacteria with a fertility inhibition system.

14. R-PLASMID Also known as Resistance plasmid. This plasmid is of great medical significance. First reported by Japanese investigators in 1959 who observed that there was sudden rise in incidence of infections caused by Shigella strains which cause epidemic of dysentry . They are also found in faeces of patients. The shigella strains are resistant simultaneously to multiple drugs; sulphonamides , streptomycin, chloramphenicol and tetracycline. These patients also showed E.coli in their faeces which are resistant to same drug.

15. R-Plasmid cont … This plasmid has two components: -resistance transfer factor(RTF) -resistance determinant(r) RTF carries genes responsible for conjugational transfer and replication. R for each of the several drugs and some of them replicate simultaneously. The entire plasmid ( RTF+r ) =R Factor. R factor contain several r determinants and resistance to eight or more drugs can be transferred by conjugation simultaneously. When two bacteria (one containing R plasmid and other devoid of R) conjugate, the R plasmid is transferred to later that lacks R plasmid.

16. STRUCTURE The structure of R plasmid is a circular piece of DNA. Length range between 80-95 kb. Constitute major portion of R-RTF molecules. This plasmid is largely homologous to F factor and contain similar genes. Contain fin 0 gene-repress function of transfer operon. R factor vary in size and in content of genes for drug resistance. R determinant is smaller than RTF. Both RTF and R determinant combine to form one unit. They are separated from each other by one IS 1 element on either side. IS 1 help in exchange of R determinant between different type of R-RTF units.

17. FUNCTIONS Important role in transmission of MDR genes in bacteria. Involved in gene carrier for autonomous replication, conjugation and resistance to ampicillin. Contain genes that can build resistance against antibiotics. Helps bacteria in producing pilli .

18. DIFFERENCE BETWEEN F AND R-PLASMID F PLASMID Has fertility factor necessary for conjugation. Has F or fertility factor. Can form sex pilus and can transfer genetic materials through conjugation. When conjugation happens between F+ and F – cells,the resultant is two F+ cells. Also, the spread of F+ cells is not a big threat. R PLASMID Has antibiotic resistance gene or resistance factor. Has R or resistance factor. Cannot form sex pilus . But, a bacterium with an F factor can pump the resistance gene to another bacterium. When R factor is transferred, it creates antibiotic resistance in bacteria. Thus, spread of R factor is a threat to antibiotic treatments.

19. COL PLASMID Also known as Colicinogenic factor. Found in several species of coliforms which produce extracellular colicins . These antibiotic like bactericidal substances are lethal toxins for other strains of same or closely related species of bacteria. Since similar antibacterial substances are also produced by bacteria other than coliforms, this group of substances are called bacteriocins . Colicins are produced by E.coli . Bacterial strains producing bacteriocin help interspecies typing of organisms.

20. Col plasmid cont … Production of colicin is determined by a plasmid known as Col factor. Resembles F factor in promoting conjugation. It also possess genetic information for self-transfer and at times, transfer of chromosomal elements.

21. STRUCTURE It is a small, closed circular plasmids. The length of Col plasmid is 6646bp. It lacks mob gene( mobility genes) and bom sequence(basis of mobility). It cannot complete the transmission independently. It contains gene colicin E1, which produce bacteriocin . It also codes for immunity against bacteriocin with imm gene.

22. FUNCTIONS It contain genes that make bacteriocin , also known as coliocin . Coliocins are proteins that kill other bacteria and thus defend host bacterium. Bacteriocins are found in many types of bacteria including E.coli , which gets them from plasmid ColE1.

23. REFERENCES Dubey , R.C.(1999). A textbook of microbiology. S.Chand and company pvt.ltd . Purohit , S.S.(2014). Microbiology fundamentals and applications. Student edition.

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