Domain architecture of a high mobility group A-type bacterial transcriptional factor.

Myxococcus xanthus transcriptional factor CarD participates in carotenogenesis and fruiting body formation. It is the only reported prokaryotic protein having adjacent "AT-hook" DNA-binding and acidic regions characteristic of eukaryotic high mobility group A (HMGA) proteins. The latter are small, unstructured, nonhistone nuclear proteins that function as architectural factors to remodel DNA and chromatin structure and modulate various DNA binding activities. We find CarD to be predominantly dimeric with two stable domains: (a) an N-terminal domain of defined secondary and tertiary structure which is absent in eukaryotic HMGA proteins; (b) a C-terminal domain formed by the acidic and AT-hook segments and lacking defined structure. CarD, like HMGA proteins, binds specifically to the minor-groove of AT-rich DNA present in two appropriately spaced tracts. As in HMGA proteins, casein kinase II can phosphorylate the CarD acidic region, and this dramatically decreases the DNA binding affinity of CarD. The acidic region, in addition to modulating DNA binding, confers structural stability to CarD. We discuss how the structural and functional plasticity arising from domain organization in CarD could be linked to its role as a general transcriptional factor in M. xanthus.