SummaryNickel serves as a cofactor for various microbial enzymes including superoxide dismutase (SOD) found in Streptomyces spp. In Streptomyces coelicolor , nickel represses and induces production of Fecontaining and Ni-containing SODs, respectively, primarily at the transcriptional level. We identified the nickel-responsive regulator (Nur), a Fur (ferric-uptake regulator) homologue, which binds to the promoter region of the sodF gene encoding FeSOD in the presence of nickel. Disruption of the nur gene caused constitutive expression of FeSOD and no induction of NiSOD in the presence of nickel. The intracellular level of nickel was higher in a D nur mutant than in the wild type, suggesting that Nur also regulates nickel uptake in S. coelicolor . A putative nickel-transporter gene cluster ( nikABCDE ) was identified in the genome database. Its transcription was negatively regulated by Nur in the presence of nickel. Purified Nur protein bound to the nikA promoter region in a nickeldependent way. These results support the action of Nur as a regulator of nickel homeostasis and antioxidative response in S. coelicolor , and add a novel nickel-responsive member to the list of versatile metal-specific regulators of the Fur family.

SummaryA gene (sigB ) encoding an alternative sigma factor s B in Streptomyces coelicolor A3 (2)

B , a homolog of stress-responsive B of Bacillus subtilis, controls both osmoprotection and differentiation in Streptomyces coelicolor A3 (2). Its gene is preceded by rsbA and rsbB genes encoding homologs of an anti-sigma factor, RsbW, and its antagonist, RsbV, of B. subtilis, respectively. Purified RsbA bound to B and prevented B -directed transcription from the sigBp1 promoter in vitro. An rsbA-null mutant exhibited contrasting behavior to the sigB mutant, with elevated sigBp1 transcription, no actinorhodin production, and precocious aerial mycelial formation, reflecting enhanced activity of B in vivo. Despite sequence similarity to RsbV, RsbB lacks the conserved phosphorylatable serine residue and its gene disruption produced no distinct phenotype. RsbV (SCO7325) from a putative six-gene operon (rsbV-rsbR-rsbS-rsbT-rsbU1-rsbU) was strongly induced by osmotic stress in a B -dependent manner. It antagonized the inhibitory action of RsbA on B -directed transcription and was phosphorylated by RsbA in vitro. These results support the hypothesis that the rapid induction of B target genes by osmotic stress results from modulation of B activity by the kinase-anti-sigma factor RsbA and its phosphorylatable antagonist RsbV, which function by a partnerswitching mechanism. Amplified induction could result from a rapid increase in the synthesis of both B and its inhibitor antagonist.Transcriptional regulators, especially sigma factors, play pivotal roles in the bacterial survival strategies such as stress responses, differentiation, social behavior, and pathogenesis (25,29). While the amounts of sigma factors are regulated through controlled synthesis and degradation, their activities are also regulated by other proteins, such as anti-sigma factors (28, 34). The activity of anti-sigma factors can be regulated by a network of other proteins, as best exemplified by the regulation of stress response and sporulation in Bacillus subtilis. There, the binding of anti-sigma factors RsbW and SpoIIAB to B and F , respectively, is regulated by their antagonist proteins, by a so called "partner-switching" mechanism, and involves interplay of various kinases and phosphatases (2,43,51).Streptomyces coelicolor is a gram-positive spore-forming soil bacterium that undergoes a complex cycle of morphological and physiological differentiation resembling that of filamentous fungi. The differentiation of S. coelicolor is dependent upon its ability to respond to changes in the environment, especially nutrient limitation, and is recognized as one of the processes to escape from the mitotic (vegetative) growth. Interplay of regulatory cascades with metabolic, morphological, homeostatic, and stress-related checkpoints has been proposed (15). Evidence for coupling differentiation with stress-related signals has accumulated in recent years. These include coregulation of stress stimulons with growth (developmental) transitions (45, 58) and participation of several stress-related sigma factors and/or their anti-sigma factors in initiating and completi...

Nickel ions serve in the correct folding and function of microbial enzymes implicated in metabolic processes. Although nickel ions are indispensable for the survival of cells, the intracellular level of nickel ions needs to be properly maintained as excessive levels of nickel ions are toxic. Nur, a nickel-uptake regulator belonging to the Fur family, is a nickel-responsive transcription factor that controls nickel homeostasis and antioxidative response in Streptomyces coelicolor. Nur was purified and crystallized at 295 K. A 2.4 Å native data set and a 3.0 Å Ni-MAD data set were collected using synchrotron radiation. The Nur crystals belong to space group P3 1 , with unit-cell parameters a = b = 78.17, c = 50.39 Å . Assuming the presence of two molecules in the asymmetric unit, the solvent content is estimated to be about 54.7%.