The Long Hunt for pssR-Looking for a Phospholipid Synthesis Transcriptional Regulator, Finding the Ribosome

The phospholipid (PL) composition of bacterial membranes varies as a function of growth rate and in response to changes in the environment. While growth adaptation can be explained by biochemical feedback in the PL synthesis pathway, recent transcriptome studies have revealed that the expression of PL synthesis genes can also be tuned in response to various stresses. We previously showed that the BasRS two-component pathway controls the expression of the diacylglycerol kinase gene, dgkA, in Escherichia coli (A. Wahl, L. My, R. Dumoulin, J. N. Sturgis, and E. Bouveret, Mol Microbiol, 80: 1260 -1275, 2011, https://doi. org/10.1111/j.1365-2958.2011.07641.x). In this study, we set up a strategy to identify the mutation responsible for the upregulation of pssA observed in the historical pssR1 mutant and supposedly corresponding to a transcriptional repressor (C. P. Sparrow and J. Raetz, J Biol Chem, 258: 9963-9967, 1983). pssA encodes phosphatidylserine synthase, the first step of phosphatidylethanolamine synthesis. We showed that this mutation corresponded to a single nucleotide change in the anti-Shine-Dalgarno sequence of the 16S rRNA encoded by the rrnC operon. We further demonstrated that this mutation enhanced the translation of pssA. Though this effect appeared to be restricted to PssA among phospholipid synthesis enzymes, it was not specific, as evidenced by a global effect on the production of unrelated proteins. IMPORTANCE Bacteria adjust the phospholipid composition of their membranes to the changing environment. In addition to enzymatic regulation, stress response regulators control specific steps of the phospholipid synthesis pathway. We wanted to identify a potential regulator controlling the expression of the phosphatidylserine synthase gene. We showed that it was not the previously suggested hdfR gene and instead that a mutation in the anti-Shine-Dalgarno sequence of 16S RNA was responsible for an increase in pssA translation. This example underlines the fact that gene expression can be modulated by means other than specific regulatory processes.