Identification of a butenolide signaling system that regulates nikkomycin biosynthesis in Streptomyces

Abstract

Butenolides are an emerging family of signaling molecules in <i>Streptomyces.</i> They control complex physiological traits, such as morphological differentiation and antibiotic production. However, how butenolides regulate these processes is poorly investigated because of obstacles in obtaining these signaling molecules. This study reports the identification of a butenolide-type signaling system for nikkomycin biosynthesis in <i>Streptomyces ansochromogenes</i> with distinct features. We identified a gene cluster, <i>sab</i>, consisting of three genes, <i>sabAPD</i>, for butenolide biosynthesis and two regulator genes, <i>sabR1</i> and <i>sabR2</i>, and characterized three butenolides (SAB1, -2, and -3) by heterologous expression of <i>sabAPD. sabA</i> disruption abolished nikkomycin production, which could be restored by the addition of SABs or by deletion of <i>sabR1</i> in ΔsabA. Electrophoretic mobility-shift assays and transcriptional analyses indicated that SabR1 indirectly represses the transcription of nikkomycin biosynthetic genes, but directly represses <i>sabA</i> and <i>sabR1</i> In the presence of SABs, the SabR1 transcriptional regulator dissociated from its target genes, verifying that SabR1 is the cognate receptor of SABs. Genome-wide scanning with the conserved SabR1-binding sequence revealed another SabR1 target gene, <i>cprC</i>, whose transcription was strongly repressed by SabR1. Intriguingly, CprC positively regulated the pleiotropic regulatory gene <i>adpA</i> by binding to its promoter and, in turn, activated nikkomycin biosynthesis. This is the first report that butenolide-type signaling molecules and their cognate receptor SabR1 can regulate <i>adpA</i> via a newly identified activator, CprC, to control nikkomycin production. These findings pave the way for further studies seeking to unravel the regulatory mechanism and functions of the butenolide signaling system in <i>Streptomyces</i>.