GrgA overexpression inhibits Chlamydia trachomatis growth through sigma66- and sigma28-dependent mechanisms

The obligate intracellular bacterium Chlamydia trachomatis is an important human pathogen with a biphasic developmental cycle comprised of an infectious elementary body (EB) and a replicative reticulate body (RB). Whereas σ⁶⁶, the primary sigma factor, is necessary for transcription of most chlamydial genes throughout the developmental cycle, σ²⁸ is required for expression of some late genes. We previously showed that the Chlamydia-specific transcription factor GrgA physically interacts with both of these sigma factors and activates transcription from σ⁶⁶- and σ²⁸-dependent promoters in vitro. Here, we investigated the organismal functions of GrgA. We show that overexpression of GrgA slows EB-to-RB conversion, decreases RB proliferation, and reduces progeny EB production. In contrast, overexpression of a GrgA variant without the σ²⁸-binding domain shows significantly less severe inhibitory effects, while overexpression of a variant without the σ⁶⁶-binding domain demonstrates no adverse effects. These findings indicate that GrgA plays important roles in the expression regulation of both σ⁶⁶-dependent genes and σ²⁸-dependent genes during the chlamydial developmental cycle.

Role of the Flagellar Hook-Length Control Protein FliK and σ28 in cagA Expression in Gastric Cell-Adhered Helicobacter pylori

Adherence of Helicobacter pylori to the gastric epithelial cell line AGS strongly induces expression of fliK encoding a flagellar hook-length control protein. FliK has a role in triggering dissociation of the alternate sigma factor, σ(28), from a nonfunctional σ(28)-FlgM complex, releasing free, functional σ(28). The σ(28)-RNA polymerase initiates transcription of cagA, the major virulence gene, from a promoter identified in this study. Consequently, significant up-regulation of cagA was observed in AGS-adhered H. pylori. Direct binding of σ(28) to the cagA promoter was demonstrated by chromatin immunoprecipitation and the transcription start site was identified by 5' RACE (rapid amplification of complementary DNA ends). The σ(28)-dependent cagA promoter was active specifically in AGS-adhered H. pylori, and this motif might be associated with high cagA expression and severity of disease. These results also indicate that H. pylori has evolved to integrate expression of the major virulence gene cagA with the flagellar regulatory circuit, essential for colonization of the human host.