Involvement of surR in silencing of the sur gene cluster. (A) RT-qPCR analysis of the effect of etoposide and ivermectin on the expression of surR. Shown is the observed fold-change in surR mRNA levels compared to the DMSO negative control. (B) HPLC-MS base-peak chromatogram analysis of the secondary metabolome of wt S. albus (gray trace), ΔsurR (black trace), ΔsurR + 1 (blue trace), and ΔsurR + 2 (red trace). (C) Targeted extracted-ion chromatograms for the products of the sur cluster in ΔsurR (black trace), ΔsurR + 1 (blue trace), and ΔsurR + 2 (red trace). Production of all three products is enhanced upon addition of 1 or 2 to the ΔsurR mutant.