Mutagenesis of region 4 of sigma 28 from Chlamydia trachomatis defines determinants for protein-protein and protein-DNA interactions

Transcription Regulation of Flagellins: A Structural Perspective

Bacterial flagella are complex molecular motors that are essential for locomotion and host colonization. They consist of 30 different proteins expressed in varying stoichiometries. Their assembly and function are governed by a hierarchical transcriptional regulatory network with multiple checkpoints primarily regulated by sigma factors. Expression of late flagellar genes requires the complete assembly of the flagellar basal body and hook. The extracellular segment of the flagellum, termed filament, is composed of self-assembling flagellin subunits encoded by the fliC gene and harbors potent antigenic epitopes. Structural studies have illuminated the molecular mechanisms underlying its assembly and its regulation at the transcription level. σ28, a key subunit of the RNA polymerase complex, binds to specific promoter sequences to initiate transcription of late flagellar genes, while its activity is controlled by the antisigma factor FlgM. This review summarizes current insights into the structural characterization of flagellins across various bacterial species, their transcription by σ28, and the structural mechanism controlling σ28 activity through FlgM. Additionally, we highlight the regulation of flagellin gene expression via transcription factors and their post-transcriptional regulation, providing a comprehensive overview of the intricate mechanisms that support bacterial motility and adaptation.

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

Chlamydia trachomatis Scc4 (formerly CT663) engages the transcription machinery and the pathogenic type III secretion system (T3SS). Both machines are required for Chlamydia infection. These requirements and the limited ability for genetic manipulation in Chlamydia 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 Escherichia coli 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 (σ⁷⁰) (homologous to chlamydial σ⁶⁶) mutants that strengthen the interaction between σ⁷⁰ region 4 and the β-flap, confirming Scc4's distinction as a module of RNAP holoenzyme capable of modulating transcription. Yersinia pestis expressing Scc4 sustains a functional T3SS, through which CopN secretion is boosted by cooption of Scc4 and Scc1. Finally, conditional expression of Scc4 in C. trachomatis results in fast expansion of the Chlamydia-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.IMPORTANCE The T3SS is a key virulence factor required for C. trachomatis 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 C. trachomatis development during infection in human epithelial cells.

Quantifying promoter activity during the developmental cycle of Chlamydia trachomatis

Chlamydia trachomatis is an important human pathogen that undergoes a characteristic development cycle correlating with stage-specific gene expression profiles. Taking advantage of recent developments in the genetic transformation in C. trachomatis, we constructed a versatile green fluorescent protein (GFP) reporter system to study the development-dependent function of C. trachomatis promoters in an attempt to elucidate the mechanism that controls C. trachomatis adaptability. We validated the use of the GFP reporter system by visualizing the activity of an early euo gene promoter. Additionally, we uncovered a new ompA promoter, which we named P3, utilizing the GFP reporter system combined with 5' rapid amplification of cDNA ends (RACE), in vitro transcription assays, real-time quantitative RT-PCR (RT-qPCR), and flow cytometry. Mutagenesis of the P3 region verifies that P3 is a new class of C. trachomatis σ(66)-dependent promoter, which requires an extended -10 TGn motif for transcription. These results corroborate complex developmentally controlled ompA expression in C. trachomatis. The exploitation of genetically labeled C. trachomatis organisms with P3-driven GFP allows for the observation of changes in ompA expression in response to developmental signals. The results of this study could be used to complement previous findings and to advance understanding of C. trachomatis genetic expression.

Multipart Chaperone-Effector Recognition in the Type III Secretion System of Chlamydia trachomatis

Secretion of effector proteins into the eukaryotic host cell is required for Chlamydia trachomatis virulence. In the infection process, Scc1 and Scc4, two chaperones of the type III secretion (T3S) system, facilitate secretion of the important effector and plug protein, CopN, but little is known about the details of this event. Here we use biochemistry, mass spectrometry, nuclear magnetic resonance spectroscopy, and genetic analyses to characterize this trimolecular event. We find that Scc4 complexes with Scc1 and CopN in situ at the late developmental cycle of C. trachomatis. We show that Scc4 and Scc1 undergo dynamic interactions as part of the unique bacterial developmental cycle. Using alanine substitutions, we identify several amino acid residues in Scc4 that are critical for the Scc4-Scc1 interaction, which is required for forming the Scc4·Scc1·CopN ternary complex. These results, combined with our previous findings that Scc4 plays a role in transcription (Rao, X., Deighan, P., Hua, Z., Hu, X., Wang, J., Luo, M., Wang, J., Liang, Y., Zhong, G., Hochschild, A., and Shen, L. (2009) Genes Dev. 23, 1818-1829), reveal that the T3S process is linked to bacterial transcriptional events, all of which are mediated by Scc4 and its interacting proteins. A model describing how the T3S process may affect gene expression is proposed.

Isolation of Chlamydia trachomatis and membrane vesicles derived from host and bacteria

The study of intracellular bacteria and nanometer-size membrane vesicles within infected host cells poses an important challenge as it is difficult to identify each distinct population in the context of the complex populations generated from active host-pathogen interactions. Here, suspension cultures of L929 cells infected with the prevalent obligate intracellular bacterium Chlamydia trachomatis strain F/Cal-IC-13 are utilized for the large scale preparation and isolation of natural membrane vesicles and bacterial forms. Cell lysis with nitrogen cavitation in combination with differential centrifugation, OptiPrep™ density gradient separation, and immunoenrichment using anti-chlamydial lipopolysaccharide antibodies and MagnaBind beads allows for the isolation of both productive and persistent bacterial forms, as well as membrane vesicles derived from the host and pathogen. We have evaluated these populations by electron microscopy and Western blot analysis for identification of biomarkers. In addition, purified persistent forms of C. trachomatis induced by ampicillin display adenosine-5'-triphosphate (ATP) transport activity, suggesting that ampicillin-induced persistent C. trachomatis organisms, at least in part, rely upon host ATP as an energy source. Importantly, several chlamydial cytotoxic and/or secreted proteins are demonstrated to be associated with these vesicles, supporting the idea that membrane vesicles are generated by Chlamydia as a means of carrying and delivering virulence factors necessary for pathogenesis. The ability to produce large-scale infections and generate distinct bacteria and host-derived populations for biochemical analysis, while reducing the burdens of time and cost have implications in all areas of chlamydiology. These protocols can be applied to other strains of C. trachomatis or other intracellular bacteria.

Altered protein secretion of Chlamydia trachomatis in persistently infected human endocervical epithelial cells

Chlamydia trachomatis is the most common bacterial infection of the human reproductive tract globally; however, the mechanisms underlying the adaptation of the organism to its natural target cells, human endocervical epithelial cells, are not clearly understood. To secure its intracellular niche, C. trachomatis must modulate the host cellular machinery by secreting virulence factors into the host cytosol to facilitate bacterial growth and survival. Here we used primary human endocervical epithelial cells and HeLa cells infected with C. trachomatis to examine the secretion of bacterial proteins during productive growth and persistent growth induced by ampicillin. Specifically, we observed a decrease in secretable chlamydial protease-like activity factor (CPAF) in the cytosol of host epithelial cells exposed to ampicillin with no evident reduction of CPAF product by C. trachomatis. In contrast, the expression of CopN and Tarp was downregulated, suggesting that C. trachomatis responds to ampicillin exposure by selectively altering the expression of secretable proteins. In addition, we observed a greater accumulation of outer-membrane vesicles from C. trachomatis in persistently infected cells. Taken together, these results suggest that the regulation of both gene expression and the secretion of chlamydial virulence proteins is involved in the adaptation of the bacteria to a persistent infection state in human genital epithelial cells.

A regulator from Chlamydia trachomatis modulates the activity of RNA polymerase through direct interaction with the beta subunit and the primary sigma subunit

The obligate intracellular human pathogen Chlamydia trachomatis undergoes a complex developmental program involving transition between two forms: the infectious elementary body (EB), and the rapidly dividing reticulate body (RB). However, the regulators controlling this development have not been identified. To uncover potential regulators of transcription in C. trachomatis, we screened a C. trachomatis genomic library for sequences encoding proteins that interact with RNA polymerase (RNAP). We report the identification of one such protein, CT663, which interacts with the beta and sigma subunits of RNAP. Specifically, we show that CT663 interacts with the flap domain of the beta subunit (beta-flap) and conserved region 4 of the primary sigma subunit (sigma(66) in C. trachomatis). We find that CT663 inhibits sigma(66)-dependent (but not sigma(28)-dependent) transcription in vitro, and we present evidence that CT663 exerts this effect as a component of the RNAP holoenzyme. The analysis of C. trachomatis-infected cells reveals that CT663 begins to accumulate at the commencement of the RB-to-EB transition. Our findings suggest that CT663 functions as a negative regulator of sigma(66)-dependent transcription, facilitating a global change in gene expression. The strategy used here is generally applicable in cases where genetic tools are unavailable.