B-184. The OmpR Subfamily Response Regulator ChxR from Chlamydia trachomatis Forms Homo-Dimers in vivo and Requires Receiver-Effector Domain Interaction for DNA-Binding
Chlamydia are obligate intracellular bacteria that are propagated through a bi-phasic developmental cycle which is intimately associated with chlamydial pathogenesis; however, the mechanisms that regulates the developmental cycle are poorly understood. An OmpR subfamily response regulator termed ChxR exhibits expression patterns supporting an important role during conversion from the non-infectious, replicative reticulate body (RB) to the non-replicative, but infectious, elementary body (EB). ChxR is a unique response regulator that is not activated through phosphorylation and prior experiments have demonstrated that unmodified ChxR is transcriptionally active. OmpR response regulators form homo-dimers as an integral step in transcriptional activation, usually as a result of phosphorylation. We hypothesize that native, unmodified ChxR forms homodimers and is maintained in a structural conformation that corresponds to the transient active state of phosphorylated OmpR response regulators. We demonstrated that a primary amine chemical cross-linker captured ChxR homodimers in vitro. We have also shown that dimer formation occurs through the amino terminal receiver domain, similar to phosphorylation-induced and transcriptionally active OmpR response regulators. Concurrently, DNA binding studies indicate the receiver domain is required for binding. Lastly, we detected ChxR dimers within Chlamydia infected eukaryotic host cells, demonstrating that ChxR forms dimers in vivo. These observations support that ChxR forms dimers and is in a constitutively active conformation.