The Bacillus subtilis DegS-DegU histidine kinase-response regulator pair controls the expression of genes encoding degradative enzymes such as levansucrase (sacB) and of genes involved in genetic competence. The mecA and mecB mutations were previously isolated as allowing competence gene expression in complex media. We have shown that the mec mutations also lead to overexpression of sacB, bypassing the DegS-DegU requirement. This expression was shown to be entirely dependent upon ComK, a positive regulator of competence gene expression. The mecB gene was cloned and its nucleotide sequence was determined. The predicted MecB protein show very high similarity over its entire length with members of the ClpC family of ATPases (60% identity). MecB is essential for growth of B. subtilis at high temperature. MecB also acts as a negative regulator of ComK synthesis, thus preventing late competence gene expression. We suggest that under these conditions MecB may interact with MecA to sequester or otherwise inactivate ComK. In response to an unknown signal, active ComK would accumulate through a positive feedback loop, leading to expression of competence genes allowing DNA uptake.