Recruitment of a novel zinc‐bound transcriptional factor by a bacterial HMGA‐type protein is required for regulating multiple processes in Myxococcus xanthus

Summary Enhanceosome assembly in eukaryotes often requires high mobility group A (HMGA) proteins. In prokaryotes, the only known transcriptional regulator with HMGA‐like physical, structural and DNA‐binding properties is Myxococcus xanthus CarD. Here, we report that every CarD‐regulated process analysed also requires the product of gene carG, located immediately downstream of and transcriptionally coupled to carD. CarG has the zinc‐binding H/C‐rich metallopeptidase motif found in archaemetzincins, but with Q replacing a catalytically essential E. CarG, a monomer, binds two zinc atoms, shows no apparent metallopeptidase activity, and its stability in vivo absolutely requires the cysteines. This indicates a strictly structural role for zinc‐binding. In vivo CarG localizes to the nucleoid but only if CarD is also present. In vitro CarG shows no DNA‐binding but physically interacts with CarD via its N‐terminal and not HMGA domain. CarD and CarG thus work as a single, physically linked, transcriptional regulatory unit, and if one exists in a bacterium so does the other. Like zinc‐associated eukaryotic transcriptional adaptors in enhanceosome assembly, CarG regulates by interacting not with DNA but with another transcriptional factor.