Conjugative plasmids of enteric bacteria from many different incompatibility groups have similar genes for single-stranded DNA-binding proteins.

Among 30 conjugative plasmids of enteric bacteria from 23 incompatibility (Inc) groups, we found 19 (from 12 Inc groups) which can complement defects caused by a defective single-stranded DNA-binding protein of Escherichia coli K-12. The genes which are responsible for the complementation from three of these plasmids (Inc groups I1, Y, and 9) were cloned. These genes showed extensive homology with each other and with the E. coli F factor ssb gene (formerly denoted ssf), which codes for a single-stranded DNA binding protein. The proteins coded for by the cloned genes bound tightly to single-stranded DNA. Six other ssb- -complementing plasmids were tested for homology to the F factor ssb gene, and all of these showed homology, as did one of the ssb- -noncomplementing plasmids. Plasmids from a total of 13 different Inc groups of enteric bacteria were found to be likely to have genes with some homology to the ssb gene of the F factor. For plasmids from several different Inc groups, we found no evidence for strong homology with ssb of the F factor.