Growing repertoire of AraC/XylS activators.

The AraC/XylS family of transcription activator proteins is defined by a 100-amino-acid region of sequence similarity (15) that forms an independently folding domain containing two helix-turn-helix (HTH) DNA binding motifs. A recent database search (http://hits.isb-sib.ch) (35) by the author using the AraC/XylS family profile (PS01124) (15) identified 830 family members! A number of AraC/XylS family members involved in carbon metabolism and responses to environmental stress have been intensively studied, including Ada, AraC, MarA, MelR, RhaR, RhaS, Rob, SoxS, and XylS (described below). More recently, several family members that regulate virulence factor expression in animal and plant pathogens have been well characterized. These virulence regulators are often part of complex regulatory schemes and are revealing new twists to regulation by the AraC/XylS family. Here I will briefly discuss some of the features of this large family and then focus on novel findings regarding AraC/XylS regulation of virulence factor expression, including those by Hulbert and Taylor in this issue (19a).