MOLECULAR-BIOLOGY OF THE LYSR FAMILY OF TRANSCRIPTIONAL REGULATORS

The LysR family is composed of >50 similar-sized, autoregulatory trans criptional regulators (LTTRs) that apparently evolved from a distant a ncestor into subfamilies found in diverse prokaryotic genera. In respo nse to different coinducers, LTTRs activate divergent transcription of linked target genes or unlinked regulons encoding extremely diverse f unctions. Mutational studies and amino acid sequence similarities of L TTRs identify: (a) a DNA-binding domain employing a helix-turn-helix m otif (residues 1-65), (b) domains involved in coinducer recognition an d/or response (residues 100-173 and 196-206), (c) a domain required fo r both DNA binding and coinducer response (residues 227-253). DNA foot printing studies suggest that in the absence of coinducer many LTTRs b ind to regulated promoters via a 15-bp dyadic sequence with a common s tructure and position (near -65). Coinducer causes additional interact ions of LTTRs with sequences near the -35 RNA polymerase binding site and/or DNA bending that results in transcription activation.