Growth of Escherichia coli K-12 on L-proline in the absence of known proline transporters

Abstract

Three transport systems have been implicated in the uptake of L-proline by the Gram-negative bacterium Escherichia coli. A triple mutant containing deletions of the putP, proP, and proW genes was constructed from strain BW25113 of the Keio collection. This mutant still grew slowly in a minimal salts medium with L-proline as the sole nitrogen source. Growth was dependent on the exogenous proline concentration but was not stimulated by 0.3 mol l-1 NaCl. It was reduced by addition of 5 mmol l-1 L-isoleucine by not by L-leucine, glycine betaine, γ- aminobutyric acid, or sarcosine. The triple mutant was partially inhibited by L-azetidine-2-carboxylate but not by 3,4-dehydro-DL-proline. Introduction of additional mutations in the proY, brnQ, or gabP genes had no effect. While the mechanism of proline uptake in this mutant is still unclear, it is likely that another membrane transporter can facilitate L-proline accumulation when the normal systems are unavailable.