# The conserved two-component systems CutRS and CssRS control the protein secretion stress response in Streptomyces

## TL;DR

This study identifies CutRS as a two-component system in Streptomyces that senses protein secretion stress and influences antibiotic production.

## Contribution

The study reveals CutRS as a secretion stress-sensing system and shows its deletion can bypass glucose-mediated repression of antibiotic biosynthesis.

## Key findings

- CutRS deletion increases growth rate and reverses glucose-mediated carbon catabolite repression.
- CutR DNA binding is glucose-dependent but does not regulate growth or antibiotic biosynthesis genes directly.
- CutRS is activated by failure of a disulfide bond between conserved cysteine residues, indicating secretion stress.

## Abstract

Streptomyces specialized metabolites account for over half of all clinically used antibiotics, as well as numerous antifungal, anticancer, and immunosuppressant agents. Two-component systems, which are widespread in bacteria, are key regulators of antibiotic production in Streptomyces species, yet their activating signals remain poorly understood. CutRS was the first two-component system identified in the genus Streptomyces, and deletion of cutRS in Streptomyces coelicolor was shown to enhance antibiotic production, although its CutR regulon does not include biosynthetic genes. Here, we used Streptomyces venezuelae NRRL B-65442 to further investigate CutRS function. We show that deletion of cutRS increases growth rate and a reversal of the glucose-mediated carbon catabolite repression typically observed in Streptomyces species. We also demonstrate that CutR DNA binding is glucose-dependent, but CutR does not directly regulate genes involved in growth, antibiotic biosynthesis, or glucose metabolism. The only CutR targets conserved in both S. coelicolor and S. venezuelae are the foldase genes htrA3 and htrB, which are involved in the protein secretion stress response. Consistent with this, we show that CutS homologs all contain two conserved cysteine residues in their extracellular sensor domains and that changing these residues to serine constitutively activates S. venezuelae CutRS. We propose that failure of a disulfide bond to form between these cysteine residues indicates secretion stress and leads to activation of the CutRS system and the secretion stress response.

Streptomyces bacteria are the primary source of clinically useful antibiotics. While many two-component systems have been linked to antibiotic biosynthesis in Streptomyces species, few have been well characterized. Here, we characterize a secretion stress-sensing two-component system called CutRS and propose a model for how the sensor kinase detects extracellular protein misfolding via two highly conserved cysteine residues. Importantly, we also show that deletion of cutRS triggers antibiotic overproduction in the presence of glucose. Since glucose normally represses antibiotic biosynthesis in Streptomyces species through carbon catabolite repression, this finding reveals a simple genetic route to bypass this barrier. This has significant implications for antibiotic discovery pipelines and industrial production, where glucose-rich media are preferred for cost and scalability. Our results position CutRS as a key target for future strain-improvement strategies.