Context-Dependent Action of Scc4 Reinforces Control of the Type III Secretion System.

Scc4 (formerly CT663) engages the transcription machinery and the pathogenic type III secretion system (T3SS). Both machines are required for infection. These requirements and the limited ability for genetic manipulation in have hampered dissection of Scc4's contributions. Here, by developing bacterial systems that permit the controlled expression and stable maintenance of Scc4, we assess Scc4's effects on chlamydial growth phenotype, secretion, and the patterns of T3SS gene expression. Expressing Scc4 in lacking a T3SS injectisome causes a growth defect. This deficiency is rescued by overexpressing the β-subunit of RNA polymerase (RNAP) or by exploiting sigma 70 (σ) (homologous to chlamydial σ) mutants that strengthen the interaction between σ region 4 and the β-flap, confirming Scc4's distinction as a module of RNAP holoenzyme capable of modulating transcription. expressing Scc4 sustains a functional T3SS, through which CopN secretion is boosted by cooption of Scc4 and Scc1. Finally, conditional expression of Scc4 in results in fast expansion of the -containing vacuole and accelerated chlamydial development, coupled to selective up- or downregulation of gene expression from different T3SS genes. This work reveals, for the first time, the context-dependent action of Scc4 linking it to diverse protein networks in bacteria. It establishes that Scc4, when overexpressed, exerts incredible effects on chlamydial development by reinforcing control of the T3SS. The T3SS is a key virulence factor required for infection. The control of the T3SS has not been well studied in this obligate intracellular pathogen. Here, we show that Scc4 plays a major role for precise control of the pathogenic T3SS at the levels of gene expression and effector secretion through genetically separable protein networks, allowing a fast adaptive mode of development during infection in human epithelial cells.