The transcriptional regulator SutA is part of a nutrient scavenging network expressed at the entry to stationary phase in Pseudomonas aeruginosa

Abstract

Bacteria in natural environments frequently encounter nutrient limitation leading to growth arrest and must balance the potential benefits of continuing to respond to the environment by making new proteins against the costs of depleting limited resources. We previously showed that the RNA polymerase-binding regulator SutA enhances transcription of hundreds of genes during nutrient limitation in Pseudomonas aeruginosa, suggesting that it might be part of a regulatory network facilitating limited new protein synthesis. Here, we sought to expand our understanding of this network by identifying transcriptional regulators influencing sutA expression. Using northern blotting, western blotting, and reporter assays, we found that the sigma factors FliA and RpoS, and the DNA-binding regulator Lrp, impact expression from a proximal sutA promoter during the transition to stationary phase. This constellation of regulators and the dynamics of SutA expression lead us to propose that SutA is part of a regulatory network that facilitates scavenging. Scavenging includes motility toward possible nutrient sources and uptake mechanisms for these nutrients, activities which require an investment of resources but can yield important benefits during starvation. In vitro transcription experiments, proteomic analysis and reporter assays suggest that SutA directly supports new protein synthesis driven by RpoS and indirectly supports flagellar motility, perhaps by helping maintain protein biosynthetic capacity against the metabolic costs of motility. SutA expression is controlled by multiple regulatory inputs, including negative autoregulation, and the protein appears to be short-lived. These properties are consistent with a role in supporting short, controlled bursts of gene expression during nutrient limitation. Author StatementMany bacteria engage in cycles of colonising a nutrient-rich location, using the available nutrients, and then dispersing in search of a new location to colonise. While searching for new nutrients in a low-resource environment, bacteria will be starved and must coordinate resource-intensive processes such as new protein synthesis, motility, and nutrient uptake so that each crucial activity can be accomplished but none use too much of the limited pool of resources. We previously identified a regulator in Pseudomonas aeruginosa called SutA, which facilitates new protein synthesis under starvation conditions. Here, we have identified regulators of SutA expression. We find that the housekeeping sigma factor RpoD drives expression during growth, but at the entry to stationary phase, where SutA has obvious impacts on cellular physiology, the stress sigma factor RpoS, the flagellar sigma factor FliA, and the amino acid sensing transcription factor Lrp are important. Finally, we find that all cells in a nutrient-limited population express some SutA, but appear to do so in infrequent bursts, and that the protein is likely unstable. Together, these findings suggest that SutA contributes to the coordination of resource use while bacteria scavenge for new nutrients, facilitating limited amounts of new protein synthesis.