RovC - a novel type of hexameric transcriptional activator promoting type VI secretion gene expression.

Type VI secretion systems (T6SSs) are complex macromolecular injection machines which are widespread in Gram-negative bacteria. They are involved in host-cell interactions and pathogenesis, required to eliminate competing bacteria, or are important for the adaptation to environmental stress conditions. Here we identified regulatory elements controlling the T6SS4 of Yersinia pseudotuberculosis and found a novel type of hexameric transcription factor, RovC. RovC directly interacts with the T6SS4 promoter region and activates T6SS4 transcription alone or in cooperation with the LysR-type regulator RovM. A higher complexity of regulation was achieved by the nutrient-responsive global regulator CsrA, which controls rovC expression on the transcriptional and post-transcriptional level. In summary, our work unveils a central mechanism in which RovC, a novel key activator, orchestrates the expression of the T6SS weapons together with a global regulator to deploy the system in response to the availability of nutrients in the species' native environment.

A bacterial secreted translocator hijacks riboregulators to control type III secretion in response to host cell contact.

Numerous Gram-negative pathogens use a Type III Secretion System (T3SS) to promote virulence by injecting effector proteins into targeted host cells, which subvert host cell processes. Expression of T3SS and the effectors is triggered upon host cell contact, but the underlying mechanism is poorly understood. Here, we report a novel strategy of Yersinia pseudotuberculosis in which this pathogen uses a secreted T3SS translocator protein (YopD) to control global RNA regulators. Secretion of the YopD translocator upon host cell contact increases the ratio of post-transcriptional regulator CsrA to its antagonistic small RNAs CsrB and CsrC and reduces the degradosome components PNPase and RNase E levels. This substantially elevates the amount of the common transcriptional activator (LcrF) of T3SS/Yop effector genes and triggers the synthesis of associated virulence-relevant traits. The observed hijacking of global riboregulators allows the pathogen to coordinate virulence factor expression and also readjusts its physiological response upon host cell contact.

Transcriptional and Post-transcriptional Regulatory Mechanisms Controlling Type III Secretion.

Type III secretion systems (T3SSs) are utilized by numerous Gram-negative bacteria to efficiently interact with host cells and manipulate their function. Appropriate expression of type III secretion genes is achieved through the integration of multiple control elements and regulatory pathways that ultimately coordinate the activity of a central transcriptional activator usually belonging to the AraC/XylS family. Although several regulatory elements are conserved between different species and families, each pathogen uses a unique set of control factors and mechanisms to adjust and optimize T3SS gene expression to the need and lifestyle of the pathogen. This is reflected by the complex set of sensory systems and diverse transcriptional, post-transcriptional and post-translational control strategies modulating T3SS expression in response to environmental and intrinsic cues. Whereas some pathways regulate solely the T3SS, others coordinately control expression of one or multiple T3SSs together with other virulence factors and fitness traits on a global scale. Over the past years, several common regulatory themes emerged, e.g., environmental control by two-component systems and carbon metabolism regulators or coupling of T3SS induction with host cell contact/translocon-effector secretion. One of the remaining challenges is to resolve the understudied post-transcriptional regulation of T3SS and the dynamics of the control process.

Adaptation to Photooxidative Stress: Common and Special Strategies of the Alphaproteobacteria Rhodobacter sphaeroides and Rhodobacter capsulatus.

Photosynthetic bacteria have to deal with the risk of photooxidative stress that occurs in presence of light and oxygen due to the photosensitizing activity of (bacterio-) chlorophylls. Facultative phototrophs of the genus Rhodobacter adapt the formation of photosynthetic complexes to oxygen and light conditions, but cannot completely avoid this stress if environmental conditions suddenly change. R. capsulatus has a stronger pigmentation and faster switches to phototrophic growth than R. sphaeroides. However, its photooxidative stress response has not been investigated. Here, we compare both species by transcriptomics and proteomics, revealing that proteins involved in oxidation-reduction processes, DNA, and protein damage repair play pivotal roles. These functions are likely universal to many phototrophs. Furthermore, the alternative sigma factors RpoE and RpoHII are induced in both species, even though the genetic localization of the rpoE gene, the RpoE protein itself, and probably its regulon, are different. Despite sharing the same habitats, our findings also suggest individual strategies. The crtIB-tspO operon, encoding proteins for biosynthesis of carotenoid precursors and a regulator of photosynthesis, and cbiX, encoding a putative ferrochelatase, are induced in R. capsulatus. This specific response might support adaptation by maintaining high carotenoid-to-bacteriochlorophyll ratios and preventing the accumulation of porphyrin-derived photosensitizers.

Metabolome and transcriptome-wide effects of the carbon storage regulator A in enteropathogenic Escherichia coli.

The carbon storage regulator A (CsrA) is a conserved global regulatory system known to control central carbon pathways, biofilm formation, motility, and pathogenicity. The aim of this study was to characterize changes in major metabolic pathways induced by CsrA in human enteropathogenic Escherichia coli (EPEC) grown under virulence factor-inducing conditions. For this purpose, the metabolomes and transcriptomes of EPEC and an isogenic ∆csrA mutant derivative were analyzed by untargeted mass spectrometry and RNA sequencing, respectively. Of the 159 metabolites identified from untargeted GC/MS and LC/MS data, 97 were significantly (fold change ≥ 1.5; corrected p-value ≤ 0.05) regulated between the knockout and the wildtype strain. A lack of csrA led to an accumulation of fructose-6-phosphate (F6P) and glycogen synthesis pathway products, whereas metabolites in lower glycolysis and the citric acid cycle were downregulated. Associated pathways from the citric acid cycle like aromatic amino acid and siderophore biosynthesis were also negatively influenced. The nucleoside salvage pathways were featured by an accumulation of nucleosides and nucleobases, and a downregulation of nucleotides. In addition, a pronounced downregulation of lyso-lipid metabolites was observed. A drastic change in the morphology in the form of vesicle-like structures of the ∆csrA knockout strain was visible by electron microscopy. Colanic acid synthesis genes were strongly (up to 50 fold) upregulated, and the abundance of colanic acid was 3 fold increased according to a colorimetric assay. The findings expand the scope of pathways affected by the csrA regulon and emphasize its importance as a global regulator.

Discovering RNA-Based Regulatory Systems for Yersinia Virulence.

The genus Yersinia includes three human pathogenic species, Yersinia pestis, the causative agent of the bubonic and pneumonic plague, and enteric pathogens Y. enterocolitica and Y. pseudotuberculosis that cause a number of gut-associated diseases. Over the past years a large repertoire of RNA-based regulatory systems has been discovered in these pathogens using different RNA-seq based approaches. Among them are several conserved or species-specific RNA-binding proteins, regulatory and sensory RNAs as well as various RNA-degrading enzymes. Many of them were shown to control the expression of important virulence-relevant factors and have a very strong impact on Yersinia virulence. The precise targets, the molecular mechanism and their role for Yersinia pathogenicity is only known for a small subset of identified genus- or species-specific RNA-based control elements. However, the ongoing development of new RNA-seq based methods and data analysis methods to investigate the synthesis, composition, translation, decay, and modification of RNAs in the bacterial cell will help us to generate a more comprehensive view of Yersinia RNA biology in the near future.

The Conserved Dcw Gene Cluster of R. sphaeroides Is Preceded by an Uncommonly Extended 5' Leader Featuring the sRNA UpsM.

Cell division and cell wall synthesis mechanisms are similarly conserved among bacteria. Consequently some bacterial species have comparable sets of genes organized in the dcw (division and cell wall) gene cluster. Dcw genes, their regulation and their relative order within the cluster are outstandingly conserved among rod shaped and gram negative bacteria to ensure an efficient coordination of growth and division. A well studied representative is the dcw gene cluster of E. coli. The first promoter of the gene cluster (mraZ1p) gives rise to polycistronic transcripts containing a 38 nt long 5' UTR followed by the first gene mraZ. Despite reported conservation we present evidence for a much longer 5' UTR in the gram negative and rod shaped bacterium Rhodobacter sphaeroides and in the family of Rhodobacteraceae. This extended 268 nt long 5' UTR comprises a Rho independent terminator, which in case of termination gives rise to a non-coding RNA (UpsM). This sRNA is conditionally cleaved by RNase E under stress conditions in an Hfq- and very likely target mRNA-dependent manner, implying its function in trans. These results raise the question for the regulatory function of this extended 5' UTR. It might represent the rarely described case of a trans acting sRNA derived from a riboswitch with exclusive presence in the family of Rhodobacteraceae.