Overexpressing the ClpC AAA+ Unfoldase Prematurely Triggers Secondary Differentiation in Chlamydia trachomatis
Chlamydia is an obligate intracellular bacteria that undergoes a complex biphasic developmental cycle, alternating between the smaller infectious non-dividing elementary body (EB) and the larger non-infectious but dividing reticulate body (RB). Due to the differences between these functionally and morphologically distinct forms, we hypothesize protein degradation is essential to chlamydial differentiation. The bacterial Clp system, consisting of an ATPase unfoldase (e.g., ClpX or ClpC) and a proteolytic component (e.g., ClpP), is critical for the physiology of bacteria through its recognition, and usually degradation, of specific substrates. We observed by transmission electron microscopy that overexpression of wild-type ClpC, but not an ATPase mutant isoform, in Chlamydia increased glycogen accumulation within the vacuolar niche of the bacteria earlier in the developmental cycle than typically observed. This suggested ClpC activity may increase expression of EB-associated genes. Consistent with this, targeted RT-qPCR analyses demonstrated a significant increase in several EB-associated gene transcripts earlier in development. These effects were not observed with overexpression of the ATPase mutant of ClpC, providing strong evidence that the activity of ClpC drives secondary differentiation. By analyzing the global transcriptional response to ClpC overexpression using RNA sequencing, we observed a global shift to earlier expression of canonical late developmental cycle genes and other EB-associated genes. Finally, we directly linked overexpression of ClpC with earlier production of EBs. Conversely, disrupting normal ClpC function with an ATPase mutant caused a delay in secondary differentiation. Overall, these findings provide the first mechanistic insight for initiation of secondary differentiation in Chlamydia. Overall design: The transformant of clpC overexpression were infected into HeLa cells. At 10 hpi, the constructs were induced or not with 20 nM aTc.  RNA samples were prepared from clpC overexpression-infected cells at 18. 20 µg RNA samples were treated with DNase to remove DNA contamination by using DNA-free Turbo kit (Thermo) according to the manusfacturer's instruction. Ribosomal RNA was depleted from samples using the MICROBEnrich (Thermo) and MICROBExpress kits (Thermo) following the manufacturer's instructions. RNA samples were processed for sequencing by the UNMC Genomics Core Facility.