Pleiotropic effects of ActVI-ORFA as an unusual regulatory factor identified in the biosynthetic pathway of actinorhodin in Streptomyces coelicolor

Regulatory networks play critical roles in controlling the the biosynthesis of natural products in Streptomyces. ActVI-ORFA, a regulatory factor encoded by the actinorhodin biosynthetic gene cluster (act cluster), positively controls the production of actinorhodin (ACT) in Streptomyces coelicolor, although its regulatory mechanism remains obscure. This study aimed to identify the regulatory targets of ActVI-ORFA. Deletion of ActVI-ORFA caused the differential expression of hundreds of proteins, as determined by two-dimensional electrophoresis and peptide mass fingerprinting analysis. qRT-PCR analysis of some genes encoding these differentially expressed proteins, including act genes and non-act genes, confirmed that ActVI-ORFA could control their transcriptional levels. In an electrophoretic mobility shift assay with a promoter region of a target gene located in the act cluster, no binding was detected, consistent with the lack of a recognizable DNA-binding domain in ActVI-ORFA. Overall, our findings suggest that ActVI-ORFA is a pleiotropic regulatory factor that controls multiple physiological pathways, including secondary metabolite production, probably via an indirect mode.