Analogs of the dnaB gene of Escherichia coli K-12 associated with conjugative R plasmids

The dnaB266(Am) mutation in Escherichia coli K-12 is an amber mutation such that strains carrying this mutation are not viable in a sup+ strain. With five different R plasmids, it has been possible to construct viable R+ derivatives of this amber mutant and show that the plasmids themselves do not carry amber suppressors. This is interpreted as evidence for the presence of dnaB analog genes associated with these plasmids. Plasmid-positive strains carrying these genes often showed some degree of cryosensitivity of DNA synthesis and colony-forming ability. These observations indicate that the presence of dnaB analog genes in association with R plasmids must be relevant to the plasmid state or to some aspect of conjugative ability.