Regulation of antimycin biosynthesis by the orphan ECF RNA polymerase sigma factor σ(AntA)

Antimycins are an extended family of depsipeptides that are made by filamentous actinomycete bacteria and were first isolated more than 60 years ago. Recently, antimycins have attracted renewed interest because of their activities against the anti-apoptotic machineries inside human cells which could make them promising anti-cancer compounds. The biosynthetic pathway for antimycins was recently characterised but very little is known about the organisation and regulation of the antimycin (ant) gene cluster. Here we report that the ant gene cluster in Streptomyces albus is organized into four transcriptional units; the antBA, antCDE, antGF and antHIJKLMNO operons. Unusually for secondary metabolite clusters, the antG and antH promoters are regulated by an extracytoplasmic function (ECF) RNA polymerase sigma factor named σ(AntA) which represents a new sub-family of ECF σ factors that is only found in antimycin producing strains. We show that σ(AntA) controls production of the unusual precursor 3-aminosalicylate which is absolutely required for the production of antimycins. σ(AntA) is highly conserved in antimycin producing strains and the −10 and −35 elements at the σ(AntA) regulated antG and antH promoters are also highly conserved suggesting a common mechanism of regulation. We also demonstrate that altering the C-terminal Ala-Ala residues found in all σ(AntA) proteins to Asp-Asp increases expression of the antFG and antGHIJKLMNO operons and we speculate that this Ala-Ala motif may be a signal for the protease ClpXP.