Sigma 54-Regulated Transcription Is Associated with Membrane Reorganization and Type III Secretion Effectors during Conversion to Infectious Forms of Chlamydia trachomatis

Chlamydia bacteria are obligate intracellular organisms with a phylum-defining biphasic developmental cycle that is intrinsically linked to its ability to cause disease. The progression of the chlamydial developmental cycle is regulated by the temporal expression of genes predominantly controlled by RNA polymerase sigma (σ) factors. Sigma 54 (σ(54)) is one of three sigma factors encoded by Chlamydia for which the role and regulon are unknown. CtcC is part of a two-component signal transduction system that is requisite for σ(54) transcriptional activation. CtcC activation of σ(54) requires phosphorylation, which relieves inhibition by the CtcC regulatory domain and enables ATP hydrolysis by the ATPase domain. Prior studies with CtcC homologs in other organisms have shown that expression of the ATPase domain alone can activate σ(54) transcription. Biochemical analysis of CtcC ATPase domain supported the idea of ATP hydrolysis occurring in the absence of the regulatory domain, as well as the presence of an active-site residue essential for ATPase activity (E242). Using recently developed genetic approaches in Chlamydia to induce expression of the CtcC ATPase domain, a transcriptional profile was determined that is expected to reflect the σ(54) regulon. Computational evaluation revealed that the majority of the differentially expressed genes were preceded by highly conserved σ(54) promoter elements. Reporter gene analyses using these putative σ(54) promoters reinforced the accuracy of the model of the proposed regulon. Investigation of the gene products included in this regulon supports the idea that σ(54) controls expression of genes that are critical for conversion of Chlamydia from replicative reticulate bodies into infectious elementary bodies.