Characterization of integration host factor (IHF) binding upstream of the cysteine-rich protein operon (omcAB) promoter of Chlamydia trachomatis LGV serovar L2

Chlamydiae are bacterial parasites that carry out a distinct developmental cycle within host cells; however, the mechanisms by which these organisms r egulate stage-specific gene expression are not known. We identified a DNA e lement located between nucleotide (nt) -135 and -90 upstream from the trans cription start point of the late stage-specific CRP operon (omcAB) of Chlam ydia trachomatis, to which a protein in extracts of chlamydiae harvested at 23 h after infection binds. A recombinant protein of C. trachomatis open r eading frame (ORF) CT267, which is homologous to bacterial integration host factor (IHF) and the heat-unstable nucleoid protein (HU), bound to the sam e element and produced the same DNase I footprint as the protein in chlamyd ial extracts. Recombinant ORF CT267 protein bound with high affinity to the DNA element and induced a sharp bend in a DNA fragment containing the bind ing site, suggesting that ORF CT267 encodes a protein with IHF-like activit y, and recombinant protein had a positive effect on in vitro transcription of the CRP operon. IHF-binding activity and IHF protein were detected in ex tracts of C. trachomatis during the early to intermediate phases of the lat e stage of the developmental cycle (between 17 and 30 h after infection), b ut were absent in the extreme late phase of the cycle and in the infectious form of chlamydiae. The presence of an IHF binding site upstream of the CR P operon and the presence of chlamydial IHF-like protein when late stage ge nes are transcribed suggests that the chlamydial IHF may play a role in sta ge-specific gene expression.