Plasmid: types, structure and functions.
- 1. PLASMIDS Submitted to Submitted by Dr. Aghil soorya Krishnapriya M Department of Botany Roll no: 10 St.Teresas college 1st 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 o Genetics o Artificial intelligence o 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.
- 6. TYPES OF PLASMIDS Based on function: PLASMIDS CONJUGATIVE NON-CONJUGATIVE
- 7. 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.
- 8. 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.
- 9. 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.
- 10. 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.
- 12. 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.
- 13. 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.
- 14. 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’.
- 16. 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.
- 17. 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.
- 18. 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.
- 19. 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.
- 21. 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.
- 22. 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.
- 23. 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.
- 24. 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.
- 25. 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.
- 27. 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.
- 28. REFERENCES 1. Dubey, R.C.(1999). A textbook of microbiology. S.Chand and company pvt.ltd. 2. Purohit, S.S.(2014). Microbiology fundamentals and applications. Student edition.
- 29. THANK YOU