Schematic model of the transcriptional regulation of sulphate reduction in C. glutamicum. The transcriptional repressor McbR acts as a top level regulator, controlling the transcription of its regulon in dependence of the effector S-adenosyl-L-homocysteine (SAH). Absence of SAH results in repression of gene transcription by active McbR while presence of SAH leads to inactivation of McbR. This allows at least basic transcription of all McbR-controlled genes, among them those encoding the regulatory proteins CysR and SsuR as well as those involved in assimilatory sulphate reduction (fpr2 cysIXHDNYZ). The second level of regulation is monitored by CysR that controls several genes in response to the availability of either O-acetyl-L-serine (OAS) or O-acetyl-L-homoserine (OAH), activating at least five transcription units while repressing two others. Among the controlled genes are those needed for assimilatory sulphate reduction, the one encoding the transcriptional regulator SsuR, and several genes of unknown function. Transcriptional regulators are represented by circles and effector metabolites are given as diamonds. Genes are displayed as boxes with transcriptional activation marked with arrows and repression marked with dashes.