PIPECOLIC ACID IS AN OSMOPROTECTANT FOR ESCHERICHIA-COLI TAKEN UP BY THE GENERAL OSMOPORTERS PROU AND PROP

Exogenously supplied L-pipecolic acid was accumulated by Escherichia c oli cells and protected them while growing at inhibitory osmolarity. U sing specific uptake mutants and competitive assays, we established th at the imino acid enters the cells through the ProP and ProU systems w ith K-m values of 225 and 53 mu M. respectively. Surprisingly, in spit e of the requirement for tire wild-type proX gene for osmoprotective a bility, no binding activity of labelled pipecolate with the periplasmi c protein encoded by proX could be detected. In an attempt to demonstr ate whether the two porters (Prop and ProU) are the only carriers invo lved in osmoregulation, a variety of molecules known for their intrace llular osmolarity-dependent accumulation in various organisms were inv estigated. N-Dimethylproline (proline betaine), N-dimethylglycine, hom obetaine (beta-aranine betaine), gamma-butyrobetaine and dimethylsulfo niopropionate were found to be capable of promoting the growth of osmo tically stressed E. coli. All of these molecules enter bacterial cells via prop and ProU porters. None of the osmoprotectants except N-dimet hylproline was able to bind the periplasmic protein encoded by ProX, w hile this protein was necessary for their uptake. Apparently, prop and ProU are the sole osmoporters involved in osmolyte influx into E. col i cells.