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

Abstract

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 SP{beta} and also contain the integrative and conjugative element ICEBs1. We found that the presence of ICEBs1 in cells inhibited production of SP{beta}, during both activation of a lysogen and following de novo infection. The ICEBs1 gene yddK (renamed spbK for SP{beta} killing) was both necessary and sufficient for the anti-SP{beta} activity. Co-expression of spbK and yonE, in the absence of other ICEBs1 and SP{beta} 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-SP{beta} phenotype protected populations of B. subtilis from predation by SP{beta}, likely providing selective pressure for the maintenance of ICEBs1 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.