Phosphate control over nitrogen metabolism in Streptomyces coelicolor: direct and indirect negative control of glnR, glnA, glnII and amtB expression by the response regulator PhoP

Bacterial growth requires equilibrated concentration of C, N and P sources. This work shows a phosphate control over the nitrogen metabolism in the model actinomycete Streptomyces coelicolor. Phosphate control of metabolism in Streptomyces is exerted by the two component system PhoR-PhoP. The response regulator PhoP binds to well-known PHO boxes composed of direct repeat units (DRus). PhoP binds to the glnR promoter, encoding the major nitrogen regulator as shown by EMSA studies, but not to the glnRII promoter under identical experimental conditions. PhoP also binds to the promoters of glnA and glnII encoding two glutamine synthetases, and to the promoter of the amtB-glnK-glnD operon, encoding an ammonium transporter and two putative nitrogen sensing/regulatory proteins. Footprinting analyses revealed that the PhoP-binding sequence overlaps the GlnR boxes in both glnA and glnII. ‘Information theory’ quantitative analyses of base conservation allowed us to establish the structure of the PhoP-binding regions in the glnR, glnA, glnII and amtB genes. Expression studies using luxAB as reporter showed that PhoP represses the above mentioned nitrogen metabolism genes. A mutant deleted in PhoP showed increased expression of the nitrogen metabolism genes. The possible conservation of phosphate control over nitrogen metabolism in other microorganisms is discussed.     

Overlapping binding of PhoP and AfsR to the promoter region of glnR in Streptomyces coelicolor

Growth of soil bacteria is often limited by the availability of essential nutrients such as carbon, nitrogen and phosphate. The reaction to a specific nutrient starvation triggers interconnected responses to equilibrate the metabolism. It is known that PhoP (response regulator involved in phosphate control) specifically binds to several promoters of genes involved in nitrogen metabolism which are also regulated by GlnR (regulator involved in nitrogen control). In this article we report a novel cross-talk between GlnR and the SARP-like regulator, AfsR. AfsR binds to some PhoP-regulated promoters including those of afsS (a small regulatory protein of secondary metabolism), pstS (a component of the phosphate transport system) and phoRP (encoding the two component system itself). We have characterized the regulation exerted upon the nitrogen regulator glnR gene by AfsR, using EMSA and DNase I footprinting assays as well as in vivo expression studies with ΔphoP, ΔafsR and ΔafsR-ΔphoP mutants. Both PhoP and AfsR proteins are able to bind to overlapping regions within the glnR promoter producing different effects. This work demonstrates a cross-talk of three different regulators of both primary and secondary metabolism.     

A complex role of <Emphasis Type="Italic">Amycolatopsis mediterranei</Emphasis> GlnR in nitrogen metabolism and related antibiotics production

Amycolatopsis, genus of a rare actinomycete, produces many clinically important antibiotics, such as rifamycin and vancomycin. Although GlnR of Amycolatopsis mediterranei is a direct activator of the glnA gene expression, the production of GlnR does not linearly correlate with the expression of glnA under different nitrogen conditions. Moreover, A. mediterranei GlnR apparently inhibits rifamycin biosynthesis in the absence of nitrate but is indispensable for the nitrate-stimulating effect for its production, which leads to the hyper-production of rifamycin. When glnR of A. mediterranei was introduced into its phylogenetically related organism, Streptomyces coelicolor, we found that GlnR widely participated in the host strain’s secondary metabolism, resemblance to the phenotypes of a unique S. coelicolor glnR mutant, FS2. In contrast, absence or increment in copy number of the native S. coelicolor glnR did not result in a detectable pleiotrophic effect. We thus suggest that GlnR is a global regulator with a dual functional impact upon nitrogen metabolism and related antibiotics production.     

Cross‐talk between two global regulators in Streptomyces: PhoP and AfsR interact in the control of afsS,pstS and phoRP transcription

The regulatory proteins AfsR and PhoP control expression of the biosynthesis of actinorhodin and undecylprodigiosin in Streptomyces coelicolor. Electrophoretic mobility shift assays showed that PhoP^DBD does not bind directly to the actII‐ORF4, redD and atrA promoters, but it binds to the afsS promoter, in a region overlapping with the AfsR operator. DNase I footprinting studies revealed a PhoP protected region of 26 nt (PHO box; two direct repeats of 11 nt) that overlaps with the AfsR binding sequence. Binding experiments indicated a competition between AfsR and PhoP; increasing concentrations of PhoP^DBD resulted in the disappearance of the AfsR–DNA complex. Expression studies using the reporter luxAB gene coupled to afsS promoter showed that PhoP downregulates afsS expression probably by a competition with the AfsR activator. Interestingly, AfsR binds to other PhoP‐regulated promoters including those of pstS (a component of the phosphate transport system) and phoRP (encoding the two component system itself). Analysis of the AfsR‐protected sequences in each of these promoters allowed us to distinguish the AfsR binding sequence from the overlapping PHO box. The reciprocal regulation of the phoRP promoter by AfsR and of afsS by PhoP suggests a fine interplay of these regulators on the control of secondary metabolism.     

Deletion of Type I glutamine synthetase deregulates nitrogen metabolism and increases ethanol production in Clostridium thermocellum

Clostridium thermocellum rapidly deconstructs cellulose and ferments resulting hydrolysis products into ethanol and other products, and is thus a promising platform organism for the development of cellulosic biofuel production via consolidated bioprocessing. While recent metabolic engineering strategies have targeted eliminating canonical fermentation products (acetate, lactate, formate, and H₂), C. thermocellum also secretes amino acids, which has limited ethanol yields in engineered strains to approximately 70% of the theoretical maximum. To investigate approaches to decrease amino acid secretion, we attempted to reduce ammonium assimilation by deleting the Type I glutamine synthetase (glnA) in an essentially wild type strain of C. thermocellum. Deletion of glnA reduced levels of secreted valine and total amino acids by 53% and 44% respectively, and increased ethanol yields by 53%. RNA-seq analysis revealed that genes encoding the RNF-complex were more highly expressed in ΔglnA and may have a role in improving NADH-availability for ethanol production. While a significant up-regulation of genes involved in nitrogen assimilation and urea uptake suggested that deletion of glnA induces a nitrogen starvation response, metabolomic analysis showed an increase in intracellular glutamine levels indicative of nitrogen-rich conditions. We propose that deletion of glnA causes deregulation of nitrogen metabolism, leading to overexpression of nitrogen metabolism genes and, in turn, elevated glutamine levels. Here we demonstrate that perturbation of nitrogen assimilation is a promising strategy to redirect flux from the production of nitrogenous compounds toward biofuels in C. thermocellum.     

Analysis of <Emphasis Type="Italic">Streptomyces coelicolor</Emphasis> M145 genes <Emphasis Type="Italic">SCO4164</Emphasis> and <Emphasis Type="Italic">SCO5854</Emphasis> encoding putative rhodaneses

Streptomyces coelicolor genome carries two apparently paralogous genes, SCO4164 and SCO5854, that encode putative thiosulfate sulfurtransferases (rhodaneses). These genes (and their presumed translation products) are highly conserved and widely distributed across actinobacterial genomes. The SCO4164 knockout strain was unable to grow on minimal media with either sulfate or sulfite as the sole sulfur source. The SCO5854 mutant had no growth defects in the presence of various sulfur sources; however, it produced significantly less amounts of actinorhodin. Furthermore, we discuss possible links between basic interconversions of inorganic sulfur species and secondary metabolism in S. coelicolor.