Sites and Systems for Conjugal DNA Transfer in Bacteria

Abstract

Plasmids isolated from Gram-negative bacteria can be divided into two major groups: large plasmids (>30 kb) that determine conjugation systems, and small plasmids (<10 kb) that do not. However, it was observed many years ago that a representative small non-conjugative plasmid, ColEl, was mobilised with high efficiency if the cell also contained an appropriate conjugative plasmid (Clowes, 1963), and more recent data for other non-conjugative plasmids suggests that this might generally be the case. Indeed, ColEl contributes not only an “origin of transfer” DNA sequence (oriT), but also mobilisation genes that are essential for its own transfer (Inselburg, 1977; Dougan and Sherratt, 1977). Interestingly, about one-third of the total plasmid DNA is devoted to conjugation, both for conjugative plasmids such as F, and for non-conjugative plasmids such as ColEl; this compares to the 5-10 fold smaller proportion required for autonomous replication.