Competence in Bacillus subtilis is expressed post-exponentially in response to signals which are interpreted by a complex network of regulatory proteins. This network culminates in the transcriptional activation of a set of late-competence proteins that mediate DNA binding and uptake during transformation. ComK, a protein that binds to competence promoters and appears to activate their transcription, is itself synthesized in response to the signal-transduction network. ComK is known to be required for the transcription of its own gene. We have placed comK under control of the xylose-inducible PxylA promoter and used this construct to show that ComK synthesis is sufficient as well as necessary to induce competence. We have also confirmed that the Mec proteins act post-transcriptionally to inactivate ComK, probably by protein-protein interaction. We have further demonstrated that ComS is required to generate an upstream signal that causes reversal of Mec-induced inactivation of ComK. In addition to ComK itself, DegU, AbrB, and SinR are required for comK transcription; mutations in their genes are bypassed by PxylA-comK induction, and therefore their products appear not to act via the Mec proteins. Overproduction of ComK, in a loss-of-function mec mutant, is also known to bypass the need for DegU, SinR and AbrB. We propose that these proteins enhance the activity of ComK as a positive autoregulatory transcription factor, acting as coactivator proteins when ComK is present at low concentrations. Finally, we demonstrate that when ComK is synthesized from the PxylA promoter and mecA is inactivated by mutation, no additional growth-stage-regulated control of competence can be detected.