Lay summary
Bacterial chromosomes consist essentially of genes that encode proteins or structural RNAs, with little chromosomal DNA left in intergenic regions, i.e. located between genes. This is a traditional view. Recent attention to intergenic regions has revealed that in fact many of them code for small regulatory RNAs. In a group of common bacteria, the pseudomonads, there might be more than a hundred such small RNAs. However, for only a handful of them are the functions known. In our research on Pseudomonas aeruginosa (a pathogen) and Pseudomonas fluorescens (a plant-protecting biocontrol agent) we are focusing on three small RNAs that regulate the synthesis of extracellular products, which act as virulence factors for P. aeruginosa and biocontrol factors for P. fluorescens. Bacterial signal molecules (“hormones”) are very important for the synthesis of these small RNAs. When bacterial populations reach a certain density, the signals are produced in sufficient quantities to turn on small RNA synthesis. On the one hand, we are trying to understand the nature of these chemically unidentified signals. On the other hand, we are studying the biochemical functions of the small RNAs. They strongly attract a class of small RNA-binding proteins whose main function is to block translation of the extracellular products mentioned. Thus, as bacterial populations grow to higher densities, signaling will favor the expression of the small RNAs and hence of the extracellular products. This process makes P. aeruginosa an aggressive pathogen and P. fluorescens a successful biocontrol agent.