Chlamydia trachomatis Type III Secretion Proteins Regulate Transcription
ABSTRACTThe Scc4 protein (CT663) of the pathogenic bacteriumChlamydiahas been described as a type III secretion (T3S) chaperone as well as an inhibitor of RNA polymerase. To examine if these roles are connected, we first investigated physical interactions betweenChlamydia trachomatisScc4 and the T3S chaperone Scc1 and a T3S substrate, CopN. In a yeast 3-hybrid assay, Scc4, Scc1, and CopN were all required to detect an interaction, which suggests that these proteins form a trimolecular complex. We also detected interactions between any two of these three T3S proteins in a pulldown assay using only recombinant proteins. We next determined whether these interactions affected the function of Scc4 as an inhibitor of RNA transcription. UsingEscherichia colias a heterologousin vivosystem, we demonstrated that expression ofC. trachomatisScc4 led to a drastic decrease in transcript levels for multiple genes. However, coexpression of Scc4 with Scc1, CopN, or both alleviated Scc4-mediated inhibition of transcription. Scc4 expression also severely impairedE. coligrowth, but this growth defect was reversed by coexpression of Scc4 with Scc1, CopN, or both, suggesting that the inhibitory effect of Scc4 on transcription and growth can be antagonized by interactions between Scc4, Scc1, and CopN. These findings suggest that the dual functions of Scc4 may serve as a bridge to link T3S and the regulation of gene expression inChlamydia.IMPORTANCEThis study investigates a novel mechanism for regulating gene expression in the pathogenic bacteriumChlamydia. TheChlamydiatype III secretion (T3S) chaperone Scc4 has been shown to inhibit transcription by RNA polymerase. This study describes physical interactions between Scc4 and the T3S proteins Scc1 and CopN. Furthermore,ChlamydiaScc1 and CopN antagonized the inhibitory effects of Scc4 on transcription and growth in a heterologousEscherichia colisystem. These results provide evidence that transcription inChlamydiacan be regulated by the T3S system through interactions between T3S proteins.