Project description:When performed at single bp resolution, the genome-wide location, occupancy level, and structural organization of DNA binding proteins provides mechanistic insights into genome regulation. Here we use ChIP-exo to provide the first high resolution view of the epigenomic organization of the E. coli transcription machinery and nucleoid structural proteins when cells are growing exponentially and upon rapid reprogramming (acute heat shock). We suggest that indirect readout of DNA shape at the flanks of cognate motifs provide major contributions to site specificity at promoter positions -35/-24 and -10/-12. We examined the site specificity of three sigma factors (RpoD/70, RpoH/32, and RpoN/54), RNA polymerase (RNAP or RpoA, B, C) and two nucleoid proteins (Fis and IHF). Our results confirm and refine reports that RpoD binds most annotated promoters, whereas RpoH and RpoN bind a much smaller subset, each through their cognate motifs. However, only upon heat shock does RpoH becomes active for RNAP recruitment. In contrast, upon heat shock RpoD remains active at its cognate promoters (including at heat shock genes), whereas RpoN remains inactive at its cognate promoters. RpoN binds ~1,000 non-annotated RpoN motifs, which may reflect a large number of condition-specific transcription units. Occupancy patterns of sigma factors and RNAP suggest a common promoter recruitment mechanism that differs from the long-standing views that sigma and RNAP are co-recruited as a complex, and also simultaneously dissociate from promoters. Our findings suggest that sigma factors are recruited and/or maintained at most promoters via an RNAP-independent mechanism. When RNAP arrives, it dwells for a relatively short time before clearing the promoter, leaving sigma behind. Taken together these findings add new dimensions to how sigma factors achieve promoter specificity through DNA sequence and shape and redefine mechanistic steps in regulated promoter assembly in E. coli.

Project description:Bacteria generally possess multiple σ factors that, based on structural and functional similarity, divide into two families: σD and σN. Among the seven σ factors in Escherichia coli, six belongs to the σD family. Each σ factor recognizes a group of promoters, providing effective control of differential gene expression. Many studies have shown that σ factors of the σD family compete with each other for function. In contrast, the competition between σN and σD families has yet to be fully explored. Here we report a global antagonistic effect on gene expression between two alternative σ factors, σN (RpoN) and σS (RpoS), a σD family protein. Mutations in rpoS and rpoN inversely affected a number of cellular traits, such as expression of flagellar genes, σN-controlled growth on poor nitrogen sources, and σS-directed expression of acid phosphatase AppA. Transcriptome analysis reveals that 40% of genes in the RpoN regulon were under reciprocal RpoS control. Furthermore, loss of RpoN led to increased levels of RpoS, while RpoN levels were unaffected by rpoS mutations. Expression of the flagellar σF factor (FliA), another σD family protein, was controlled positively by RpoN but negatively by RpoS. These findings unveil a complex regulatory interaction among σN, σS and σF, and underscore the need to employ systems biology approaches to assess the effect of such interaction of σ factors on cellular functions, including motility, nutrient utilization, and stress response. Precise deletion mutants of rpoS or rpoN of MG1655 were constructed and employed in this study. Cultures were inoculated in triplicate in M9 minimal media (0.2% glucose) at a starting OD of 0.0001 and grown aerobically at 37C. Cultures were harvested at OD600 = 0.3 in exponential phase. For RNA extraction, cultures were mixed directly with a boiling lysis buffer containing SDS and EDTA followed by acidic hot phenol to minimize RNA degradation. RNA samples were hybridized to Affymetrix E. coli Genome 2.0 Array according to Affymetrix's standard protocols.

Project description:The RpoN-RpoS alternative sigma factor pathway is essential for key adaptive responses by Borrelia burgdorferi, particularly those involved in the infection of a mammalian host. A putative response regulator, Rrp2, ostensibly is required for activation of the RpoN-dependent transcription of rpoS. However, questions remain regarding the extent to which the three major constituents of this pathway (Rrp2, RpoN, and RpoS) act interdependently. To assess the functional interplay between Rrp2, RpoN, and RpoS, we employed microarray analyses to compare gene expression levels in rrp2, rpoN, or rpoS mutants of parental strain 297. We identified 98 genes that were similarly regulated by Rrp2, RpoN and RpoS, and an additional 47 genes were determined to be likely regulated by this pathway. The substantial overlap between genes regulated by RpoS and RpoN provides compelling evidence that these two alternative sigma factors form a congruous pathway and that RpoN regulates B. burgdorferi gene expression through RpoS. Although several known B. burgdorferi virulence determinants were regulated by the RpoN-RpoS pathway, a defined function has yet to be ascribed to most of the genes substantially regulated by Rrp2, RpoN and RpoS, therefore providing additional avenues for future research into the genetic determinants contributing to virulence expression by B. burgdorferi. Keywords: gene profiling 297 (wt) vs rpoS mutant: 6 slides, 12 hybridizations (including dyeswaps), three biological replicates 297 (wt) vs rpoN mutant: 5 slides, 10 hybridizations (including dyeswaps), three biological replicates 297 (wt) vs rrp2 mutant: 6 slides, 12 hybridizations (including dyeswaps), three biological replicates 5A18 (wt) vs XY424.3 (rrp2 mutant): 5 slides, 10 hybridizations (including dyeswaps), three biological replicates 297 (wt) vs rpoS mutant: 6 slides, 12 hybridizations (including dyeswaps), three biological replicates 297 (wt) vs rpoN mutant: 5 slides, 10 hybridizations (including dyeswaps), three biological replicates 297 (wt) vs rrp2 mutant: 6 slides, 12 hybridizations (including dyeswaps), three biological replicates 5A18 (wt) vs XY424.3 (rrp2 mutant): 5 slides, 10 hybridizations (including dyeswaps), three biological replicates

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Escherichia coli str. K-12 substr. MG1655

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