The role of proline in osmoregulation in Salmonella typhimurium and Escherichia coli.

Previously we demonstrated that high intracellular proline levels conferred enhanced osmotolerance on Salmonella typhimurium. Stimulation of growth rate under conditions of osmotic inhibition could be brought about either by the addition of proline to the culture media or by mutations which result in proline over-production. Here, we report that there is a novel proline transport system in S. typhimurium which is activated in media of elevated osmolarity. this conclusion is based on the observation that putP- proP- double mutants, which lack both of the known proline permeases are resistant in minimal medium to the toxic proline analogues 3,4-dehydro-D, L-proline and L-azetidine-2-carboxylate. However, they regain sensitivity to the analogues in media of elevated osmolarity. Selecting resistance to 3,4-dehydro-D, L-proline in media of elevated osmolarity, we obtained mutants of putP- proP- strains which lack the osmotically activated proline permease. In these strains, exogenous proline could no longer alleviate osmotic inhibition.