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

Myxococcus xanthus transcriptional factor CarD participates in carotenogene sis and fruiting body formation. It is the only reported prokaryotic protei n having adjacent "AT-hook" DNA-binding and acidic regions characteristic o f eukaryotic high mobility group A (HMGA) proteins. The latter are small, u nstructured, nonhistone nuclear proteins that function as architectural fac tors to remodel DNA and chromatin structure and modulate various DNA bindin g activities. We find CarD to be predominantly dimeric with two stable doma ins: (a) an N-terminal domain of defined secondary and tertiary structure w hich is absent in eukaryotic HMGA proteins; (b) a C-terminal domain formed by the acidic and AT-hook segments and lacking defined structure. CarD, lik e HMGA proteins, binds specifically to the minor-groove of AT-rich DNA pres ent in two appropriately spaced tracts. As in HMGA proteins, casein kinase II can phosphorylate the CarD acidic region, and this dramatically decrease s the DNA binding affinity of CarD. The acidic region, in addition to modul ating DNA binding, confers structural stability to CarD. We discuss how the structural and functional plasticity arising from domain organization in C arD could be linked to its role as a general transcriptional factor in M. x anthus.