An integrative and conjugative element encodes an abortive infection system to protect host cells from predation by a bacteriophage

Bacteriophages and integrative and conjugative elements (ICEs, a.k.a. conjugative transposons) are mobile genetic elements that can move from one bacterial cell to another and often coexist in a host genome. Many bacterial species contain at least one temperate (lysogenic) phage and an integrative and conjugative element. Most strains of Bacillus subtilis are lysogenic for the phage SPß and also contain the integrative and conjugative element ICE Bs1 . We found that the presence of ICE Bs1 in cells inhibited production of SPß, during both activation of a lysogen and following de novo infection. The ICE Bs1 gene yddK ( renamed spbK for SPß killing) was both necessary and sufficient for the anti-SPß activity. Co-expression of spbK and yonE , in the absence of other ICE Bs1 and SPß genes, resulted in inhibition of cell growth and loss of cell viability. These results indicate that together spbK and yonE affected a host process needed for normal growth and that spbK constitutes an abortive infection system. We found that this anti-SPß phenotype protected populations of B. subtilis from predation by SPß, likely providing selective pressure for the maintenance of ICE Bs1 in B. subtilis populations . spbK encodes a TIR (Toll-interleukin-1 receptor)-domain protein with similarity to both plant antiviral proteins and animal innate immune signaling proteins. We postulate that selective many uncharacterized cargo genes in ICEs may confer selective advantage to cells by protecting against other mobile elements.