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