Synthesis, Release, and Recapture of Compatible Solute Proline by Osmotically Stressed Bacillus subtilis Cells

Abstract

ABSTRACTBacillus subtilissynthesizes large amounts of the compatible solute proline as a cellular defense against high osmolarity to ensure a physiologically appropriate level of hydration of the cytoplasm and turgor. It also imports proline for this purpose via the osmotically inducible OpuE transport system. Unexpectedly, anopuEmutant was at a strong growth disadvantage in high-salinity minimal media lacking proline. Appreciable amounts of proline were detected in the culture supernatant of theopuEmutant strain, and they rose concomitantly with increases in the external salinity. We found that the intracellular proline pool of severely salinity-stressed cells of theopuEmutant was considerably lower than that of itsopuE+parent strain. This loss of proline into the medium and the resulting decrease in the intracellular proline content provide a rational explanation for the observed salt-sensitive growth phenotype of cells lacking OpuE. None of the known MscL- and MscS-type mechanosensitive channels ofB. subtilisparticipated in the release of proline under permanently imposed high-salinity growth conditions. The data reported here show that the OpuE transporter not only possesses the previously reported role for the scavenging of exogenously provided proline as an osmoprotectant but also functions as a physiologically highly important recapturing device for proline that is synthesizedde novoand subsequently released by salt-stressedB. subtiliscells. The wider implications of our findings for the retention of compatible solutes by osmotically challenged microorganisms and the roles of uptake systems for compatible solutes are considered.