A conserved anti-repressor controls horizontal gene transfer by proteolysis.

The mobile genetic element ICEBs1 is an integrative and conjugative element (a conjugative transposon) found in the Bacillus subtilis chromosome. The SOS response and the RapI-PhrI sensory system activate ICEBs1 gene expression, excision and transfer by inactivating the ICEBs1 repressor protein ImmR. Although ImmR is similar to many characterized phage repressors, we found that, unlike these repressors, inactivation of ImmR requires an ICEBs1-encoded anti-repressor ImmA (YdcM). ImmA was needed for the degradation of ImmR in B. subtilis. Coexpression of ImmA and ImmR in Escherichia coli or co-incubation of purified ImmA and ImmR resulted in site-specific cleavage of ImmR. Homologues of immR and immA are found in many mobile genetic elements. We found that the ImmA homologue encoded by B. subtilis phage phi105 is required for inactivation of the phi105 repressor (an ImmR homologue). ImmA-dependent proteolysis of ImmR repressors may be a conserved mechanism for regulating horizontal gene transfer.