We previously cloned the sigH gene encoding a stress-response sigma factor sigma(H) in Streptomyces coelicolor A3(2), located in an operon with the gene encoding proposed anti-sigma factor UshX. To clarify the in vivo function of sigma(H), a stable null mutant of sigH was prepared by homologous recombination. This mutation appeared to have no obvious effect on vegetative growth, but dramatically affected morphological differentiation. Microscopy showed that the sigH mutant produced undifferentiated hyphae with rare spore chains, giving the colony a pale gray color compared to the dark gray wild-type spores. The sigH mutation partially affected growth under conditions of high osmolarity. Expression of the sigH operon was investigated in the S. coelicolor sigH mutant. Out of four promoters directing expression of the sigH operon, the sigH-P2 promoter--the only promoter preferentially induced by salt-stress conditions--was inactive in the sigH mutant. The results indicated that the sigH-P2 promoter is dependent (directly or indirectly) upon sigma(H) and that the operon is autocatalytically activated. We propose that in S. coelicolor sigma(H) has a dual role, regulating the osmotic response and morphological differentiation.
Mithramycin A is an antitumor compound used for treatment of several types of cancer including chronic and acute myeloid leukemia, testicular carcinoma, hypercalcemia and Paget's disease. Selective modifications of this molecule by combinatorial biosynthesis and biocatalysis opened the possibility to produce mithramycin analogues with improved properties that are currently under preclinical development. The mithramycin A biosynthetic gene cluster from Streptomyces argillaceus ATCC12956 was cloned by transformation assisted recombination in Saccharomyces cerevisiae and heterologous expression in Streptomyces lividans TK24 was evaluated. Mithramycin A was efficiently produced by S. lividans TK24 under standard fermentation conditions. To improve the yield of heterologously produced mithramycin A, a collection of derivative strains of S. lividans TK24 were constructed by sequential deletion of known potentially interfering secondary metabolite gene clusters using a protocol based on the positive selection of double crossover events with blue pigment indigoidine-producing gene. Mithramycin A production was evaluated in these S. lividans strains and substantially improved mithramycin A production was observed depending on the deleted gene clusters. A collection of S. lividans strains suitable for heterologous expression of actinomycetes secondary metabolites were generated and efficient production of mithramycin A with yields close to 3 g/L, under the tested fermentation conditions was achieved using these optimized collection of strains.
SummaryAs free-living non-motile saprophytes, Streptomyces need to adapt to a wide range of environmental conditions and this is reflected by an enormous diversity of regulatory proteins encoded by, for example, the genome of the model streptomycete Streptomyces coelicolor. In this organism, we have identified a new osmoregulation gene, osaC, encoding a member of a novel family of regulatory proteins. Members of the family have a predicted domain composition consisting of an N-terminal kinase domain related to antisigma factors, sensory Pas and Gaf domains, and a C-terminal phosphatase domain. osaC is linked to the response regulator gene osaB; expression analysis of the latter revealed that it is induced after osmotic stress in a s B -dependent manner. OsaC is required to return osaB and sigB expression back to constitutive levels after osmotic stress. From analysis of the activities of OsaCDPho, lacking the C-terminal phosphatase domain, and OsaCN92A, with a substitution of a critical asparagine residue in the kinase domain, we infer that this N-terminal domain functions as a s B anti-sigma factor. Indeed, co-purification experiments indicate association of OsaC and s B . These results support a model for post-osmotic stress modulation of s B activity by OsaC.
By using a previously established method for the identification of promoters recognized by a particular sigma factor of RNA polymerase, we identified a promoter in Streptomyces coelicolor A3(2) that is recognized by a heterologous RNA polymerase containing the late sporulation-specific sigma factor sigma(F). The promoter directed the expression of a gene named ssgB, which is related to the sporulation-specific cell division gene ssgA. These genes, together with three others, constitute a new family of paralogous genes specific for Streptomyces. S1-nuclease mapping using RNA prepared from an Escherichia coli strain that expresses ssgB under the control of sigma(F), and from S. coelicolor A3(2) at various developmental stages, identified identical transcription start points in both strains, corresponding to the promoter ssgBp. The promoter is developmentally regulated in S. coelicolor: it is induced at the time of aerial mycelium formation and is most active during sporulation. However, the level of the ssgB transcript was unaffected in a sigF mutant of S. coelicolor A3(2). Interestingly, the level of the transcript was substantially reduced in an S. coelicolor strain that was mutant for the sigH gene, which encodes a stress-response sigma factor (sigma(H)) that is essential for sporulation in S. coelicolor A3(2). This dependence of ssgB upon sigH was confirmed by an in vitro transcription assay, in which sigma(H), in the presence of the S. coelicolor core RNA polymerase, was able to recognize ssgBp. These results suggest that the S. coelicolor ssgB gene is under the control of the stress-response sigma(H). Transcription of ssgB was investigated in S. coelicolor A3(2) mutants with lesions in each of six known early whi genes required for sporulation septation. Expression of ssgB was deregulated in three of the mutants ( whiA, whiI, and whiJ). Based on these data, it is proposed that the ssgB gene product plays a role in the developmental process in S. coelicolor A3(2).
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