Conjugation is a process in which the genetic information (DNA) is transferred to, recipient bacteria by direct contact between them through establishment of cytoplasmic bridge.
In this process Donor bacteria posses fertility plasmids
(F plasmid )or sex plasmids that are self transmissible. They exist in both gram positive and gram negative bacteria.
F plasmids are transferred with in strains of same species.
A recipient cell that has received DNA as a result of conjugation is called trans-conjugant or ex- conjugant.
Promiscuous Plasmid
Generally plasmid transfer is intraspecific, but many plasmid transfer DNA between unrelated species, called promiscuous plasmid.
Types of Conjugation
• F+– F− Conjugation
• Hfr – F− Conjugation
• F' – F− Conjugation
F+ – F− conjugation :
F plasmid (F-factor) in E.coli
E.coli strains with extra chromosomal F plasmid are called F+ and act as donors.
Strains that lack F plasmids are F−, act as recipient.
F− plasmid spread by infection among genetically compatible populations of bacteria.
F Plasmid :
F plasmid Consist of 25 transfer genes. That function for expression of sex pili, synthesise & transfer of DNA interfere with F+ ability to serve as recipient
These transfer gene has two type
1). (Mpf) Mating pair formation gene.
2). (Dtr) DNA transfer gene (plasmid).
Steps of F+ and F− Conjugation :
1) Formation of 1 to 3 sex pili of F+ bacteria bind to specific outer membrane protein on recipient bacteria.
2) Intracellular cytoplasmic bridge is formed.
3) Relaxase Protein makes a single-strand cut at ori T(Origin of Transfer) site in Plasmid.
4) Transfer of one strand from F+ cell to F− cell, simultaneously F− plasmid replicated in F+ cell.
5) Completion of transfer - Transferred strand in converted to double strand in recipient and both the exconjugant bacteria are becomes F+.
Hfr – F− Conjugation :
Hfr : (High frequency recombination strains) Hfr strains are strains which has F plasmid attached with it's Chromosomes.
An E.coli strain with integrated F Plasmids that retain its ability to function as donor in conjugal mutation.
Integrated F plasmid transfer genes with high efficiency.
Steps of Hfr - F− conjugation :
1) Transfer or single stranded DNA from Hfr donor to F- recipient start at ori T.
2) A part of F plasmid is transferred lost after complete bacterial genome.
3) complete transfer takes 100 min and conjugation takes before this process ends.
Thus complete F plasmid is not transferred and exconjugant remain Hfr and F-.
After donor Chromosome enter recipient it may be degraded or incorporated by Homologous recombination.
F' - F− Conjugation :
F' Plasmid : Integrated F plasmids in Hfr strains can sometimes excised from bacterial Chromosomes. In rare cases excision occurs by recombinations involving insertion sequences or other genes on bacterial Chromosome to become integrated into hybrid F plasmid.Due to error in excision of F− Plasmin from Hfr the chromosomal gene of donor is picked up.
This gene is transferred to recipient and it become partially diploid merozygote. As it has 2 sets of genes carried by plasmid and need not be incorporated in genome of bacteria.
