Analysis of the DNA-binding domain of the Pseudomonas aeruginosa quorum sensing transcription factor LasR
Many bacteria engage in quorum sensing (QS), a cell-cell communication system used to coordinate group behaviors. In one type of QS, acyl-homoserine lactone signals generated by LuxI homologs bind to LuxR homolog transcription factors, usually resulting in gene activation. The genome of Pseudomonas aeruginosa encodes three such LuxR homologs: LasR, RhlR, and QscR. Of these, LasR regulates the most genes, including that encoding RhlR. There is strong evidence that, during chronic infections, lasR and other genes encoding LuxR-type regulators are under strong selective pressure for mutations that both inactivate and modulate their function. Thus, we wondered if some mutations in the lasR gene might result in a protein with affinity for promoters usually regulated by the other LuxR homologs; to do so, we investigated the DNA-binding domain (DBD) of LasR through alanine substitution. As expected, we found that most alanine substitutions across the LasR DBD led to loss of function, as did previously identified clinical LasR DBD variants. Additionally, some alanine mutants were indistinguishable from the wild type. We describe a handful of variant LasR polypeptides that unexpectedly exhibit enhanced regulation on a RhlR-regulated gene, which conferred a fitness defect when competed against the wild type. Most other LasR variants had a competitive advantage. Our results suggest a pathway for expansion of the regulon of LuxR-homolog transcription factors, but also that such mutations may be disfavored due to the incurred metabolic burden. Importance Many bacteria generate chemical signals to alter gene expression in response to changes in population density, a phenomenon called quorum sensing. One type of quorum sensing relies on acyl-homoserine lactone (AHL) signals. In this type of quorum sensing, first described in the bioluminescent bacterium Vibro fisheri, a LuxI homolog produces the AHL, which binds to a LuxR homolog that typically activates gene expression. The opportunistic pathogen Pseudomonas aeruginosa has two such LuxR homologs, LasR and RhlR, each of which has its own specific regulon. We focused on the transcription factor LasR and investigated structural determinants of its binding to target promoters using an alanine substitution approach. We discovered that some DNA-binding mutations can expand the range of LasR-regulated genes. Our work provides insight into understanding what promoters LuxR homologs bind to and, more generally, how these proteins might evolve over time to change the group of genes that they regulate.