Ectopic expression of transcriptional regulatory proteins reveals two classes of late genes during chlamydial development, corresponding to the IB and EB cell type.
The bacteria in the chlamydiales order are obligate intracellular parasites of eukaryotic cells. They are reliant on an infectious cycle consisting of at least three phenotypically distinct cell forms termed the reticulate body (RB), the intermediate body (IB) and the elementary body (EB). The EB is infectious but does not replicate. The RB replicates in the host cell but is non-infectious, while the IB is an intermediate form that transitions to the EB form. Within this order, the genus Chlamydia contains the causative agents of a number of important pathogens of humans. C. psittaci causes zoonotic infections resulting in pneumonia, while C. pneumoniae is a human pathogen that causes respiratory disease and is linked to atherosclerosis. Biovars of C. trachomatis are the causative agents of trachoma, the leading cause of preventable blindness worldwide, as well as sexually transmitted infections with the potential to cause pelvic inflammatory disease and infertility. Irrespective of the resulting disease, all chlamydial species share the same obligate intracellular life cycle and developmental cell forms. In this study we investigated the role of four transcriptional regulators on chlamydial gene expression and completion of the developmental cycle. We ectopically expressed the transcriptional repressor Euo, each of the two nucleoid associated proteins HctA and HctB and the two component sensor kinase CtcB in the RB. All four blocked development of infectious EBs. Transcriptional analysis using RNA-seq and differential expression clustering revealed three classes of gene expression. FISH analysis demonstrated that these classes corresponded to distinct cell forms revealing the gene expression profile of the RB, IB and EB developmental forms. Additionally, these data revealed that the genes for the T3SS were cell type restricted. The structural genes for the T3SS were expressed primarily in the IB while the two translocons were expressed in distinct cell types one in the RB and the other in the EB. Overall these data show that the chlamydial developmental cycle produces three distinct regulons corresponding to the RB, IB and EB cell forms and that the T3SS is expressed in a cell form specific manner suggesting defined functional roles for the T3SS in specific cell forms. Overall design: RNA-seq analuysis of Chlamydia trachomatis ectopicaly expressing the regulatory proteins, Euo, HctA, CtcB and HctB