DNA PROCESSING REACTIONS IN BACTERIAL CONJUGATION

Bacterial conjugation is an important source of genetic plasticity. The initiation complex for conjugative transfer of transmissible plasmids—the relaxosome—is a specific DNA-protein structure that has been isolated from cells and reconstituted from purified components in vitro. Complexes containing uncleaved DNA and DNA cleaved at the nicsite in the origin of transfer (oriT) coexist in equilibrium. Relaxase is usually loaded onto oriT by accessory DNA-binding proteins. Relaxase catalyzes cleavage of a specific phosphodiester bond at nic and becomes covalently linked through a tyrosyl residue to the 5′ terminus of the cleaved strand. Cleaved DNA may be unwound for transfer by a plasmid-encoded helicase. Single-strand transfer is thought to occur by a replicative rolling circle mechanism. Termination of a round of transfer is achieved by the cleaving-joining activity of the relaxase linked to the 5′ end of the transferring strand. Relationships between DNA processing reactions and conjugative interactions of cell envelopes are particularly obscure aspects of the conjugation cycle.