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)1 proteins. The latter are small, unstructured, non-histone 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.