STEPWISE ASSEMBLY OF A RELAXOSOME AT THE F PLASMID ORIGIN OF TRANSFER

A central step in the transfer of genetic information during bacterial conjugation of the Escherichia coli F plasmid involves the formation of a protein-DNA complex, called the relaxosome, at the origin of tran sfer. During conjugation, the relaxosome introduces a site- and strand -specific nick from which the physical transfer of a single strand of DNA is initiated. At least two F-encoded proteins, TraIp (tra gene pro duct) and TraYp (traY gene product), and one host-encoded protein, int egration host factor, are involved in this process. In this report, we use DNase I protection and electron microscopic techniques to investi gate the mechanism of relaxosome formation. Our results show that TraY p and integration host factor form a protein-DNA complex that facilita tes the binding of TraIp to assemble a relaxosome capable of introduci ng a site- and strand-specific nick at the origin of transfer. This ni ck is identical to that observed during conjugation.