Differential expression of the seven rRNA operon promoters from the plant growth-promoting bacterium Pseudomonas sp. UW4

Bacteria often have multiple copies of ribosomal RNA (rrn) genes in their genomes. The presence of multiple rrn operons suggests an advantage to the organism, perhaps through adjustable control of protein expression in response to altered environmental conditions. In the work described here, the strengths of the seven rRNA promoters of Pseudomonas sp. UW4 were individually assessed by separately cloning each promoter region into an expression vector and monitoring the activity of the reporter protein, the Escherichia coli lacZ gene product. The lacZ expression was the highest for the rrnE promoter under all growth conditions, with the various promoters demonstrating a range of strengths. These findings indicate that these promoters are not functionally identical. This observation suggests that the differential expression of rrn operons under various physiological conditions and growth stages allows better regulation of rRNA, conferring an advantage to P. sp. UW4 through a more fine-tuned control of protein expression in a wide range of environmental situations.