Requirements for suppression of a dnaG mutation by an I-type plasmid.

We have confirmed and further characterized the phenomenon of suppression of a dnaG mutation by an I plasmid and determined the requirements for the complete expression of suppressibility. The authentic derepressed mutation of the conjugal fertility system, described earlier, was shown to be one of the requirements. The second requirement is a previously undescribed type of mutation leading to a far higher degree of derepression of this system than the authentic drd mutation. This second mutation leads to extremely high conjugal fertility and pilus production. A third requirement for the complete expression of suppressibility is that the host recA function remain intact. recA function is required not merely for the fulfillment of the other two requirements described above, but also for an additional step or steps leading to its complete expression. The nature of this step(s) is unknown, but it is not insertion of the plasmid into the host genome.