The regulatory action of the myxobacterial CarD/CarG complex: a bacterial enhanceosome?

A global regulatory complex made up of two unconventional transcriptional factors, CarD and CarG, is implicated in the control of various processes in Myxococcus xanthus, a Gram-negative bacterium that serves as a prokaryotic model system for multicellular development and the response to blue light. CarD has a unique two-domain architecture composed of: (1) a C-terminal DNA-binding domain that resembles eukaryotic high mobility group A (HMGA) proteins, which are relatively abundant, nonhistone components of chromatin that remodel DNA and prime it for the assembly of multiprotein-DNA complexes essential for various DNA transactions, and (2) an N-terminal domain involved in interactions with CarG and RNA polymerase, which is also the founding member of the large CarD_TRCF family of bacterial proteins. CarG, which does not bind DNA directly, has a zinc-binding motif of the type found in the archaemetzincin class of metalloproteases that, in CarG, appears to play a purely structural role. This review aims to provide an overview of the known molecular details and insights emerging from the study of the singular CarD-CarG prokaryotic regulatory complex and its parallels with enhanceosomes, the higher order, nucleoprotein transcription complexes in eukaryotes.