DNA-independent transport of plasmid primase protein between bacteria by the L1 conjugation system

Abstract

The Collb-P9 (Incl1)-encoded conjugation system supports transfer of the plasmid T-strand plus hundreds of molecules of the Sog polypeptides determined by the plasmid primase gene. Here, we report that Sog primase is abundantly donated to the recipient cell from cells carrying a non-transferable Collb plasmid deleted of the nic site essential for DNA export. Such DNA-independent secretion of Sog primase is typical of authentic conjugation, both in being blocked when the recipient cell specifies the entry exclusion function of Collb and in requiring the thin I1 pilus encoded by the Collb pil system under the mating conditions used. It is proposed that Sog polypeptides form a complex with the Collb T-strand during conjugation and aid DNA transport through processive secretion of the proteins into the recipient cell. Functional and genetic relationships between the Collb conjugation system and other type IV secretion pathways are discussed.