Abstract
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 Bacillus subtilis. We demonstrate that pLS20 contains an exclusion system and identified the single gene responsible for exclusion, named sespLS20, which is embedded in the conjugation operon. SespLS20 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 sespLS20 expression, and that sespLS20 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.
Highlights
Horizontal gene transfer (HGT) refers to the transfer of DNA regions from one cell to another which happens at large scale in bacteria
We show that SespLS20 is a surface-located protein and that it is the prototype of a new family of SE proteins encoded by multiple conjugative elements present in G+ bacteria, including pathogens
Most information available on exclusion systems is related to conjugative plasmids replicating in G− bacteria, prompting us to study the SE system of the Gram+ B. subtilis conjugative plasmid pLS20
Summary
Horizontal gene transfer (HGT) refers to the transfer of DNA regions from one cell to another which happens at large scale in bacteria (for review see, Ochman et al, 2000; Frost et al, 2005; Thomas and Nielsen, 2005). HGT plays a major role in the evolution of bacteria, but is responsible for the emergence and dissemination of antibiotic resistance (Mazel and Davies, 1999). Gene 27 encodes the master regulator of conjugation, RcopLS20, which binds to the operator sites located near the divergently oriented promoters Pc and Pr, the latter driving expression of rcopLS20. This binding results in repression of Pc promoter and activation of the Pr promoter thereby keeping conjugation in the default “OFF” state (Singh et al, 2013; Ramachandran et al, 2014). In this study we addressed the question whether pLS20cat contains an exclusion system and what role(s) it plays in conjugation
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