Conditionally positive effect of the TetR-family transcriptional regulator AtrA on streptomycin production by Streptomyces griseus.

AtrA, a transcriptional activator for actII-ORF4, encoding the pathway-specific transcriptional activator of the actinorhodin biosynthetic gene cluster in Streptomyces coelicolor A3(2), has been shown to bind the region upstream from the promoter of strR, encoding the pathway-specific transcriptional activator of the streptomycin biosynthetic gene cluster in Streptomyces griseus [Uguru et al. (2005) Mol Microbiol 58, 131-150]. The atrA orthologue (atrA-g) in S. griseus was constitutively transcribed throughout growth from a promoter located about 250 nt upstream of the translational start codon, as determined by S1 nuclease mapping. DNase I footprinting showed that histidine-tagged AtrA-g bound an inverted repeat located upstream of strR at positions -117 to -142 relative to the transcriptional start point of strR as +1. This AtrA-g-binding site was between two AdpA-binding sites at approximately nucleotide positions -270 and -50. AdpA is a central transcriptional activator in the A-factor regulatory cascade and essential for the transcription of strR. AtrA-g and AdpA simultaneously bound the respective binding sites. In contrast to AdpA, AtrA-g was non-essential for strR transcription; an atrA-g-disrupted strain produced streptomycin on routine agar media to the same extent as the wild-type strain. However, the atrA-g-disrupted strain tended to produce a smaller amount of streptomycin than the wild-type strain under some conditions, for example, on Bennett agar containing 1 % maltose and on a minimal medium. Therefore, AtrA-g had a conditionally positive effect on streptomycin production, as a tuner, probably by enhancing the AdpA-dependent transcriptional activation of strR in a still unknown manner.