CRP and FNR are well-characterized members of the CRP/FNR family of regulatory proteins that function to maximize cellular energy production. In this study, we identify several new subgroups of the CRP/FNR family, many of which have not yet been characterized.

The CRP/FNR family of regulatory proteins constitutes a large collection of related transcription factors, several of which globally regulate cellular energy production. A well-characterized example is FNR (called FnrL in Rhodobacter capsulatus ), which is responsible for regulating the expression of numerous genes that promote maximal energy production and growth under anaerobic conditions.

In Rhodobacter capsulatus, the histidine kinase RegB is believed to phosphorylate its cognate transcriptional factor RegA only under anaerobic conditions. However, transcriptome evidence indicates that RegA regulates 47 genes involved in energy storage, energy production, signaling and transcription, under aerobic conditions. In this study, we provide evidence that RegA is a copper binding protein and that copper promotes the dimerization of RegA under aerobic conditions. Inductively coupled plasma mass spectrometry (ICP-MS) analysis indicates that RegA binds Cu1+ and Cu2+ in a 1:1 and 2:1 ratio, respectively. Through LC-MS/MS, ESI-MS and non-reducing SDS-PAGE gels, we show that Cu2+ stimulates disulfide bond formation in RegA at Cys156 in the presence of oxygen. Finally, we used DNase I footprint analysis to demonstrate that Cu2+-mediated covalent dimerized RegA is capable of binding to the ccoN promoter, which drives the expression of cytochrome cbb3 oxidase subunits. This study provides a new model of aerobic regulation of gene expression by RegA involving the formation of an intermolecular disulfide bond.