Final full-text version of the paper available at: http://nar.oxfordjournals.org/cgi/content/abstract/18/8/2149 .-- Copyright © by Oxford University Press. -- Http://nar.oxfordjournals.org/The XylS family consists of at least 8 different transcriptional regulators. Six of these proteins are positive regulators for the catabolism of carbon sources (benzoate and sugars) in Escherichia coli, Pseudomonas putida and Erwinia carotovora, and two of them are involved in pathogenesis in Escherichia coli and Yersinia enterocolitica. Based on protein alignments, the members of this family exhibit a long stretch of homology at the C-terminal end. The regulators involved in the catabolism of carbon sources stimulate transcription from their respectively regulated promoters only in the presence of effectors. In two of the regulators, mutations at the non-homologous Nterminus alter affinity and specificity for effectors while mutations at the conserved C-terminus part decrease activation of transcription from their corresponding regulated promoters. It is thus probable that the variable N-terminus end in this family of regulators contains the motif involved in effector recognition, while the C-terminal end is involved in DNA-binding. These proteins seem to be related by common ancestry and may act through similar mechanisms of positive regulation effected through similar folding patterns.Work in Granada was supported by a grant from the Comision Interministerial de Ciencia y Tecnologia (BT0023/87) to J.L.R.; L.Z. was supported by a post-doctoral fellowship from the Spanish Ministry of Education and F.R. is a post-doctoral fellow of the Juan March Foundation.Peer reviewed