| Summary: | Most bacterial genomes contain horizontally acquired and transmissible mobile genetic elements, including temperate bacteriophages and integrative and conjugative elements. Little is known about how these elements interact and co-evolved as parts of their host genomes. In many cases, it is not known what advantages, if any, these elements provide to their bacterial hosts. Most strains of <jats:italic>Bacillus subtilis</jats:italic> contain the temperate phage SPß and the integrative and conjugative element ICE<jats:italic>Bs1</jats:italic>. Here we show that the presence of ICE<jats:italic>Bs1</jats:italic> in cells protects populations of <jats:italic>B</jats:italic>. <jats:italic>subtilis</jats:italic> from predation by SPß, likely providing selective pressure for the maintenance of ICE<jats:italic>Bs1</jats:italic> in <jats:italic>B</jats:italic>. <jats:italic>subtilis</jats:italic>. A single gene in ICE<jats:italic>Bs1</jats:italic> (<jats:italic>yddK</jats:italic>, now called <jats:italic>spbK</jats:italic> for SPß killing) was both necessary and sufficient for this protection. <jats:italic>spbK</jats:italic> inhibited production of SPß, during both activation of a lysogen and following <jats:italic>de novo</jats:italic> infection. We found that expression <jats:italic>spbK</jats:italic>, together with the SPß gene <jats:italic>yonE</jats:italic> constitutes an abortive infection system that leads to cell death. <jats:italic>spbK</jats:italic> encodes a TIR (Toll-interleukin-1 receptor)-domain protein with similarity to some plant antiviral proteins and animal innate immune signaling proteins. We postulate that many uncharacterized cargo genes in ICEs may confer selective advantage to cells by protecting against other mobile elements.
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