Surface Exclusion Revisited: Function Related to Differential Expression of the Surface Exclusion System of Plasmid pLS20.

During conjugation a genetic element is transferred from a bacterial donor to a recipient cell via a connecting channel. It is the major route responsible for the spread of antibiotic resistance. Conjugative elements can contain exclusion system(s) that inhibit its transfer to a cell already harboring the element. Our limited knowledge on exclusion systems is mainly based on plasmids of Gram-negative bacteria. Here we studied the conjugative plasmid pLS20 of the Gram-positive . We demonstrate that pLS20 contains an exclusion system and identified the single gene responsible for exclusion, named , which is embedded in the conjugation operon. Ses is the founding member of a novel family of surface exclusion proteins encoded by conjugative elements of Gram-positive origin. We show that the extent of surface exclusion correlates with the level of expression, and that is expressed at basal low-levels in all donor cells but becomes highly expressed in conjugating cells. Accordingly, the transfer of pLS20 from a conjugation-primed donor cell to an un-primed or conjugation-primed donor is inhibited moderately and very efficiently, respectively. The consequences of this differential regulation, which appears to be a conserved feature of surface exclusion systems of Gram-positive and Gram-negative origin, are discussed.