A third L-proline permease in Salmonella typhimurium which functions in media of elevated osmotic strength

Abstract

Exogenous proline specifically stimulates the growth rate of enteric bacteria in media of inhibitory osmotic strength. It was observed that S. typhimurium mutants which lack both of the previously known proline permeases (putP proP) are stimulated by proline in media of inhibitory osmolarity. It is proposed that there is a third proline permease which functions only in media of elevated osmolarity. This conclusion is based on the observations that, in media of elevated osmolarity, the sensitivity of putP proP mutants to toxic proline analogs increases, proline requirements for maximal growth of proline auxotrophic putP proP mutants decreases, and the specific rate of incorporation of radioactive proline into protein of growing cells increases. A Tn10-induced mutation in a gene (proU) required for the functioning of the third proline permease was obtained and the map location was determined to be at 59 map units of the chromosome, between srlA and tct, 66% linked to nalB in P22 transduction. The results suggest that the function of the third, osmotically stimulated permease might be to accumulate high intracellular proline levels during osmotic stress. Possible mechanisms by which proline might cause growth stimulation are discussed.