ChxR is a transcriptional activator in Chlamydia

Chlamydia spp. are obligate intracellular bacterial pathogens that alternate between two metabolically and morphologically distinct developmental forms, and differentiation depends on transcriptional regulation. Genome sequencing of Chlamydia trachomatis revealed an ORF, CT630 (chxR), whose amino acid sequence contains a winged helix–turn–helix motif similar to the DNA-binding domain of response regulators in the OmpR subfamily. ChxR differs from many response regulators in that essential residues in the receiver or phosphorylation domain are lacking. ChxR functions as a transcriptional regulator because it activated transcription of ompF and ompC when expressed in Escherichia coli. In vitro transcription combined with microarray analysis also demonstrated that ChxR activates its own expression by binding directly to sites upstream of chxR; it also activates infA, tufA, oppA, and CT084. DNase I protection studies showed that ChxR bound to sites in the ompF and ompC promoter proximal regions that overlap but were distinct from OmpR binding sites. Both proteins could bind simultaneously to their nonoverlapping binding sites. This report identifies a stage-specific transcriptional regulator and some of its target genes in Chlamydia

Cell cycle regulator phosphorylation stimulates two distinct modes of binding at a chromosome replication origin

In Caulobacter crescentus, the global response regulator CtrA controls chromosome replication and determines the fate of two different cell progenies. Previous studies proposed that CtrA represses replication by binding to five sites, designated [a–e], in the replication origin. We show that phosphorylated CtrA binds sites [a–e] with 35- to 100–fold lower K(d) values than unphosphorylated CtrA. CtrA phosphorylation stimulates two distinct modes of binding to the replication origin. Phosphorylation stimulates weak intrinsic protein–protein cooperation between half-sites and does not stimulate CtrA–P binding unless protein–DNA contacts are made at both half-sites. CtrA phosphorylation also stimulates cooperative binding between complete sites [a] and [b]. However, binding to each of the other CtrA-binding sites [c], [d] and [e] is completely independent and suggests a modular organization of replication control by CtrA. We therefore propose a model where a phosphorelay targets separate biochemical activities inside the replication origin through both cooperative and independent CtrA-binding sites