ScnR1-Mediated Competitive DNA Binding and Feedback Inhibition Regulate Guvermectin Biosynthesis in Streptomyces caniferus

Guvermectin, a <i>Streptomyces</i>-derived purine nucleoside compound, exhibits dual bioactivities as a plant growth regulator and an antibacterial agent. While its biosynthetic gene cluster (BGC) is regulated by the cluster-situated activator GvmR and the adjacent repressor GvmR2, the role of distal transcriptional regulators (TRs) in guvermectin biosynthesis remains unexplored. Here, we identified ScnR1, a highly conserved LacI-family TR located far from the guvermectin BGC, which is directly activated by GvmR. Overexpression of <i>scnR1</i> significantly suppressed guvermectin biosynthesis. Further investigations revealed that ScnR1 competitively binds to the <i>gvmR</i>, <i>gvmA</i>, and <i>O1</i> promoters (overlapping with the GvmR-binding sites), thereby inhibiting the guvermectin BGC transcription. Moreover, ScnR1 formed a reciprocal feedback loop with the adjacent repressor GvmR2, where each repressor inhibits the other's expression. These findings reveal a multi-layered regulatory mechanism wherein LacI-family TRs fine-tune guvermectin biosynthesis through competitive DNA binding and reciprocal feedback control. This study offers new perspectives on the hierarchical control of secondary metabolism in <i>Streptomyces</i> and provides valuable theoretical guidance for the engineering of strains with enhanced natural product production.