The bifunctional role of aconitase in Streptomyces viridochromogenes Tü494

Abstract

In many organisms, aconitases have dual functions; they serve as enzymes in the tricarboxylic acid cycle and as regulators of iron metabolism. In this study we defined the role of the aconitase AcnA in Streptomyces viridochromogenes Tü494, the producer of the herbicide phosphinothricyl-alanyl-alanine, also known as phosphinothricin tripeptide or bialaphos. A mutant in which the aconitase gene acnA was disrupted showed severe defects in morphology and physiology, as it was unable to form any aerial mycelium, spores nor phosphinothricin tripeptide. AcnA belongs to the iron regulatory proteins (IRPs). In addition to its catalytic function, AcnA plays a regulatory role by binding to iron responsive elements (IREs) located on the untranslated region of certain mRNAs. A mutation preventing the formation of the [4Fe-4S] cluster of AcnA eliminated its catalytic activity, but did not inhibit RNA-binding ability. In silico analysis of the S. viridochromogenes genome revealed several IRE-like structures. One structure is located upstream of recA, which is involved in the bacterial SOS response, and another one was identified upstream of ftsZ, which is required for the onset of sporulation in streptomycetes. The functionality of different IRE structures was proven with gel shift assays and specific IRE consensus sequences were defined. Furthermore, RecA was shown to be upregulated on post-transcriptional level under oxidative stress conditions in the wild-type strain but not in the acnA mutant, suggesting a regulatory role of AcnA in oxidative stress response.