Positive and negative regulation of GlnR in validamycin A biosynthesis by binding to different loci in promoter region

Validamycin A (VAL-A) is a C7N aminocyclitol antibiotic produced by Streptomyces hygroscopicus var. jinggangensis 5008, which has been widely used as antifungal agent against rice sheath blight disease. VAL-A biosynthesis has been proven to be affected by gamma-butyrolactone and temperature. Herein, we showed that GlnR, a global regulator in nitrogen metabolism, is specifically associated with valK-valA intergenic promoter region by DNA-affinity chromatography and MS-based protein identification. Subsequent EMSA and DNase I footprinting assays revealed two GlnR binding sites in this promoter region. Targeted disruption of glnR in S. hygroscopicus 5008 led to a significant increase in the transcription of VAL-A structural genes, albeit the VAL-A production was reduced by 80 % and the sporulation of the mutant was impaired. Compared with the wild-type 5008, site-specific mutagenesis of GlnR binding site I enhanced VAL-A production by 2.5-fold, whereas the mutation of GlnR binding site II resulted in a 50 % reduction of VAL-A yield. Moreover, tandem mutation of site I in the site II mutant led to a 66 % increase of VAL-A production. The result suggested that GlnR not only serves as an inhibitor by binding site I but also as an activator by binding site II for VAL-A biosynthesis. Furthermore, overexpression of glnR in the site I mutant JG45 improved VAL-A production for 41 % compared with the control strain containing the vector. Therefore, the obtained data illustrate a novel regulatory feature of the global regulator GlnR. GlnR is firstly proved to act simultaneously as an activator and a repressor in validamycin biosynthesis by binding to different loci within a promoter region of the gene cluster.