proP-mediated proline transport also plays a role in Escherichia coli osmoregulation.

The growth of Escherichia coli strains in media having elevated osmolarity was promoted in the presence of low concentrations of the L-proline analog 5-hydroxy-L-pipecolic acid. The osmoprotective ability of this compound was correlated with the presence in these strains of a functional proP+ gene. The results suggest that the proP-mediated transport of L-proline (in addition to that by proU) is important in osmoregulation. proP::lac operon fusions were used to demonstrate that this gene shows limited induction of expression upon growth in media having elevated osmolarity and that it is transcribed clockwise on the chromosome.