Characterization of ProQ: An RNA binding protein modulating expression of the osmosensor and osmoregulator ProP of Escherichia coli
The ability to both sense and respond to variations in osmolality is essential for the survival of organisms inhabiting environments of extreme or varying osmolalities. ProP is an H+-osmoprotectant symporter that transports osmoprotectants into 'Escherichia coli' Following osmotic upshifts. In the absence of ProQ, the osmotic activation of ProP is reduced in amplitude as ProQ acts to amplify ProP activity at a post-transcriptional level. ProQ contains two distinct domains. The N-terminal domain of ProQ has a circular dichroism (CD) spectrum characteristic of alpha-helical structure, and through homology can be modeled on the crystal structure of the alpha-helical mRNA-binding protein FinO. The C-terminal domain of ProQ has a CD spectrum characteristic of beta sheet structure and can be modeled on the structure of the RNA chaperone Hfq. Functional characterization of these domains shows that the N-terminal domain of ProQ alone is able to partially complement a ' proQ' deletion. Full amplification of ProP activity is not seen, however, unless both the N- and C-terminal domains of ProQ are expressed. Deletion mutations within the 'proU' open reading frame, which encodes a second osmoregulatory transporter in 'E. coli' with a similar substrate specificity to ProP, suppressed the 'proQ' phenotype. Further analysis of this phenomenon revealed that suppression occurs in the absence of the 'proU' coding sequence, rather than in the absence of ProU transport activity. Observations presented here suggest that ProQ acts to amplify 'proP' expression by interacting with a small non-coding RNA (sRNA) and that this sRNA is either encoded within, or regulated by the expression of the 'proU' operon.