Examining the effects of site-specific mutations in the Universal Stress Protein A of Escherichia coli

Like all organisms that live in variable environments, bacteria must sense and respond to environmental changes in order to survive. The Escherichia coli Universal Stress Protein A is upregulated in response to many of these environmental changes, including oxidative stress, heat shock, and genotoxic stress and is phosphorylated on a serine or threonine in response to growth arrest. In this study, we utilized a library of mutants in which highly conserved serines and threonines in uspA have been substituted with alanines or aspartates. We used these mutant strains and plasmids containing stress promoter::luxCDABE fusions to assess the effects of these substitutions on the transcriptional activation of the katG, grpE, and recA promoters. We identified two residues (S44 and T48) that, when mutated to either an alanine or an aspartate, prevent the activation of katG. The Sl17A substitution severely decreased the overall transcription ofthe grpE and recA promoters in both stressed and unstressed cells, but this deficiency was rescued with the S117D substitution. Thus, serine-117 is a highly likely site of phosphorylation. We also found that the UspA knock-out and Sl17A mutants have significantly less total RNA than wild-type E. coli. A fluorescence in situ hybridization further showed that these two mutant strains have much less rRNA than wild-type, suggesting that both mutants have fewer ribosomes than wild-type cells. Thus, we propose that UspA may be involved in controlling ribosome turnover