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Home >> Bacterial Recombination >>Molecular Mechanism of Conjugation
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Molecular Mechanism of Conjugation - The mechanism of DNA transfer during conjugation is complex and well coordinated. Proteins traY and traI nick one strand of the P+ factor or Hfr chromosome at a specific locus called origin of transfer (ori1).

The 5' -end of the nicked strand progressively separates from its complementary strand and is transferred into the F- cell t!"1rough the conjugation tube traY/traI, multimer protein binds to oriT after nicking and facilities the separation of the DNA molecule.

The separation of the single stranded DNA from the complementary strand is thought to be coupled with rolling circle replication model. Surprisingly, only one unit length of F factor is transferred from an F+ to F- cell even when it is linked with rolling circle replication, which can produce concatemers or multiple copies of the DNA molecule.

The mechanism is clearly not understood. About 1200 bp per second are separated and is independent of DNA replication. The separated single strand is transferred into the F- cell with the 5' end first. The single strand transferred into the F- cell serves as a template for the synthesis of its complementary strand.

Conjugation is controlled by a 33 kb region of the F plasmid called transfer region (tr). This region contains -40 genes, which are named as tra and trb loci. These genes are arranged in 3 transcriptional units (traJ, traM and traY-I). tral encodes regulator or repressor protein, which control the expression of traM and tra r-- I. Gene finP codes a regulator antisense RNA that turns off traf.

Interestingly gene finP is under the control of another gene finO. Gene traA encodes the monomeric protein pilin, which is assembled into F -pili. The assembly of F pili is governed by at least 12 tra genes.

Conjugation begins when the tip of F -pilus of a cell comes in contact with F­ cell. Interaction of F-pili with F+/Hfr cells is prevented by two genes, traS and tra T, which encode for surface exclusion proteins that make the cell a poor recipient for such contacts.

Once the mating is initiated, pili disassemble, the conjugating cells come closer and a conjugation bridge is formed by traD gene products. Protein tram recognizes the formation of mating pair, following which tra Y binds near oriT. This causes traI to bind and nic oriT at a unique site called nic. traI is a relaxase (helicase activity).

 

 
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