Direct proteolytic control of an extracytoplasmic function RNA polymerase sigma factor

Abstract

The survival of any microbe relies upon its ability to respond to environmental change. Use of Extra Cytoplasmic Function (ECF) RNA polymerase sigma ({sigma}) factors is a major strategy enabling dynamic responses to extracellular signals. Streptomyces species harbor a large number of ECF {sigma} factors; nearly all of which regulate genes required for morphological differentiation and/or response to environmental stress, except for {sigma}AntA, which regulates starter-unit biosynthesis in the production of antimycin, an anticancer compound. Unlike a canonical ECF {sigma} factor, whose activity is regulated by a cognate anti-{sigma} factor, {sigma}AntA is an orphan, raising intriguing questions about how its activity may be controlled. Here, we reconstitute in vitro ClpXP proteolysis of {sigma}AntA, but not a variant lacking a C-terminal di-alanine motif. Furthermore, we show that the abundance of {sigma}AntA in vivo is enhanced by removal of the ClpXP recognition sequence, and that levels of the protein rise when cellular ClpXP protease activity is abolished. These data establish direct proteolysis as an alternative and thus far unique control strategy for an ECF RNA polymerase {sigma} factor and expands the paradigmatic understanding of microbial signal transduction regulation. ImportanceNatural products produced by Streptomyces species underpin many industrially- and medically-important compounds. However, the majority of the ~30 biosynthetic pathways harboured by an average species are not expressed in the laboratory. This undiscovered biochemical diversity is believed to comprise an untapped resource for natural products drug discovery. A major roadblock preventing the exploitation of unexpressed biosynthetic pathways is a lack of insight into their regulation and limited technology for activating their expression. Our findings reveal that the abundance of {sigma}AntA, which is the cluster-situated regulator of antimycin biosynthesis, is controlled by the ClpXP protease. These data link proteolysis to the regulation of natural product biosynthesis for the first time and we anticipate that this will emerge as a major strategy by which actinobacteria regulate production of their natural products. Further study of this process will advance understanding of how expression of secondary metabolism is controlled and will aid pursuit of activating unexpressed biosynthetic pathways.