Abstract
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 σAntA which represents a new sub-family of ECF σ factors that is only found in antimycin producing strains. We show that σAntA controls production of the unusual precursor 3-aminosalicylate which is absolutely required for the production of antimycins. σAntA is highly conserved in antimycin producing strains and the −10 and −35 elements at the σAntA 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 σAntA 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.
Highlights
60% of the antibiotics and anticancer compounds currently used in human medicine are derived from the secondary metabolites of soil-dwelling Streptomyces species and other filamentous actinomycetes
We report that σ AntA is regulated at the transcriptional level and controls production of the unusual precursor 3-aminosalicylate that is required for antimycin production
We identified two overlapping cosmids containing antC and confirmed that cosmid 213 contains the complete predicted ant gene cluster by deleting genes adjacent to the cluster using PCR-targeted mutagenesis (Fig. 1)
Summary
60% of the antibiotics and anticancer compounds currently used in human medicine are derived from the secondary metabolites of soil-dwelling Streptomyces species and other filamentous actinomycetes. We show that σ AntA represents a new sub-family of ECF σ factors that are only found in the ant gene clusters of Streptomyces species and provide evidence that suggests σ AntA regulation of the divergent antGF and antHIJKLMNO operons is conserved in all antimycin producing strains.
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