DNA-independent transport of plasmid primase protein between bacteria by the I1 conjugation system

The ColIb-P9 (IncI1)-encoded conjugation system supports transfer of the pl asmid T-strand plus hundreds of molecules of the Sog polypeptides determine d by the plasmid primase gene. Here, we report that Sog primase is abundant ly donated to the recipient cell from cells carrying a non-transferable Col Ib plasmid deleted of the nic site essential for DNA export. Such DNA-indep endent secretion of Sog primase is typical of authentic conjugation, both i n being blocked when the recipient cell specifies the entry exclusion funct ion of ColIb and in requiring the thin I1 pilus encoded by the ColIb pil sy stem under the mating conditions used. It is proposed that Sog polypeptides form a complex with the ColIb T-strand during conjugation and aid DNA tran sport through processive secretion of the proteins into the recipient cell. Functional and genetic relationships between the ColIb conjugation system and other type IV secretion pathways are discussed.