Regulation of Horizontal Gene Transfer in Bacillus subtilis by Activation of a Conserved Site-Specific Protease

The mobile genetic element ICEBs1 is an integrative and conjugative element (a conjugative transposon) found in Bacillus subtilis. The RecA-dependent SOS response and the RapI-PhrI cell sensory system activate ICEBs1 gene expression by stimulating cleavage of ImmR, the ICEBs1 immunity repressor, by the protease ImmA. We found that increasing the amount of wild-type ImmA in vivo caused partial derepression of ICEBs1 gene expression. However, during RapI-mediated derepression of ICEBs1 gene expression, ImmA levels did not detectably increase, indicating that RapI likely activates the protease ImmA by increasing its specific activity. We also isolated and characterized mutations in immA (immA(h)) that cause partial derepression of ICEBs1 gene expression in the absence of inducing signals. We obtained two types of immA(h) mutations: one type caused increased amounts of the mutant proteins in vivo but no detectable effect on specific activity in vitro; the other type had no detectable effect on the amount of the mutant protein in vivo but caused increased specific activity of the protein (as measured in vitro). Together, these findings indicate that derepression of ICEBs1 gene expression is likely caused by an increase in the specific activity of ImmA. Homologs of ImmA and ImmR are found in many mobile genetic elements, so the mechanisms that regulate ImmA-mediated cleavage of ImmR may be widely conserved.