Impact of MtrA on phosphate metabolism genes and the response to altered phosphate conditions in Streptomyces

Phosphate metabolism is known to be regulated by the PhoPR regulatory system in Streptomyces and some other bacteria. In this study, we report that MtrA also regulates phosphate metabolism in Streptomyces. Our data showed that, in Streptomyces coelicolor, MtrA regulates not only phosphate metabolism genes such as phoA but also phoP under different phosphate conditions, including growth on rich complex media without added inorganic phosphate and on defined media with low or high concentrations of inorganic phosphate. Cross-regulation was also observed among mtrA, phoP and glnR under these conditions. We demonstrated both in vitro and in vivo binding of MtrA to the promoter regions of genes associated with phosphate metabolism and to the intergenic region between phoR and phoU, indicating that these phosphate metabolism genes are targets of MtrA. We further showed that MtrA in S. lividans and S. venezuelae has detectable regulatory effects on expression of phosphate metabolism genes. Additionally, the MtrA homologue from Corynebacterium glutamicum bound predicted MtrA sites of multiple phosphate metabolism genes, implying its potential for regulating phosphate metabolism in this species. Overall, our findings support MtrA as a major regulator for phosphate metabolism in Streptomyces and also potentially in other actinobacteria.     

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.     

Proteomic analysis of the GlnR-mediated response to nitrogen limitation in Streptomyces coelicolor M145

GlnR is the global regulator of nitrogen assimilation in Streptomyces coelicolor M145 and other actinobacteria. Two-dimensional polyacrylamide gel electrophoresis analyses were performed to identify new GlnR target genes by proteomic comparison of wild-type S. coelicolor M145 and a ΔglnR mutant. Fifty proteins were found to be differentially regulated between S. coelicolor M145 and the ΔglnR mutant. These spots were identified by nanoHPLC–ESI-MS/MS and classified according to their cellular role. Most of the identified proteins are involved in amino acid biosynthesis and in carbon metabolism, demonstrating that the role of GlnR is not restricted to nitrogen metabolism. Thus, GlnR is supposed to play an important role in the global metabolic control of S. coelicolor M145.     

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.     

A lipoprotein modulates activity of the MtrAB two‐component system to provide intrinsic multidrug resistance,cytokinetic control and cell wall homeostasis in Mycobacterium

The MtrAB signal transduction system, which participates in multiple cellular processes related to growth and cell wall homeostasis, is the only two‐component system known to be essential in Mycobacterium. In a screen for antibiotic resistance determinants in Mycobacterium smegmatis, we identified a multidrug‐sensitive mutant with a transposon insertion in lpqB, the gene located immediately downstream of mtrA–mtrB. The lpqB mutant exhibited increased cell–cell aggregation and severe defects in surface motility and biofilm growth. lpqB cells displayed hyphal growth and polyploidism, reminiscent of the morphology of Streptomyces, a related group of filamentous Actinobacteria. Heterologous expression of M. tuberculosis LpqB restored wild‐type characteristics to the lpqB mutant. LpqB interacts with the extracellular domain of MtrB, and influences MtrA phosphorylation and promoter activity of dnaA, an MtrA‐regulated gene that affects cell division. Furthermore, in trans expression of the non‐phosphorylated, inactive form of MtrA in wild‐type M. smegmatis resulted in phenotypes similar to those of lpqB deletion, whereas expression of the constitutively active form of MtrA restored wild‐type characteristics to the lpqB mutant. These results support a model in which LpqB, MtrB and MtrA form a three‐component system that co‐ordinates cytokinetic and cell wall homeostatic processes.     

Atypical OmpR/PhoB Subfamily Response Regulator GlnR of Actinomycetes Functions as a Homodimer,Stabilized by the Unphosphorylated Conserved Asp-focused Charge Interactions

The OmpR/PhoB subfamily protein GlnR of actinomycetes is an orphan response regulator that globally coordinates the expression of genes related to nitrogen metabolism. Biochemical and genetic analyses reveal that the functional GlnR from Amycolatopsis mediterranei is unphosphorylated at the potential phosphorylation Asp⁵⁰ residue in the N-terminal receiver domain. The crystal structure of this receiver domain demonstrates that it forms a homodimer through the α4-β5-α5 dimer interface highly similar to the phosphorylated typical response regulator, whereas the so-called “phosphorylation pocket” is not conserved, with its space being occupied by an Arg⁵² from the β3-α3 loop. Both in vitro and in vivo experiments confirm that GlnR forms a functional homodimer via its receiver domain and suggest that the charge interactions of Asp⁵⁰ with the highly conserved Arg⁵² and Thr⁹ in the receiver domain may be crucial in maintaining the proper conformation for homodimerization, as also supported by molecular dynamics simulations of the wild type GlnR versus the deficient mutant GlnR(D50A). This model is backed by the distinct phenotypes of the total deficient GlnR(R52A/T9A) double mutant versus the single mutants of GlnR (i.e. D50N, D50E, R52A and T9A), which have only minor effects upon both dimerization and physiological function of GlnR in vivo, albeit their DNA binding ability is weakened compared with that of the wild type. By integrating the supportive data of GlnRs from the model Streptomyces coelicolor and the pathogenic Mycobacterium tuberculosis, we conclude that the actinomycete GlnR is atypical with respect to its unphosphorylated conserved Asp residue being involved in the critical Arg/Asp/Thr charge interactions, which is essential for maintaining the biologically active homodimer conformation.