The Activity and Specificity of the Proline Permease in Wild-type and Analogue-resistant Strains of Escherichia coli

Abstract

SUMMARY: The main characteristics of the previously described proline-specific transport mechanism (permease) of Escherichia coli were confirmed in strain C4. The same permease was responsible for entry of a number of proline analogues, including 3,4-dehydroproline, 4-methyleneproline, cis- and trans-4-chloroprolines, thiazolidine-4-carboxylic acid (thioproline) and the lower homologue, azetidine-2-carboxylic acid. These analogues also entered the cells by an exchange reaction between extracellular analogue and previously accumulated intracellular proline. Growth of the parent (C4) strain was inhibited by 3,4-dehydroproline and azetidine-2-carboxylic acid, both of which were incorporated into cellular protein. Several classes of mutants, selected for resistance to either dehydroproline or azetidine, failed to incorporate one or both analogues into protein. Some of these mutants owed their resistance to failure to produce a functional proline permease. At least one strain, resistant to azetidine but not to dehydroproline, possessed an altered permease with little affinity for azetidine-2-carboxylic acid but still capable of transporting proline and 3,4-dehydroproline; the permease of this strain could no longer promote exchange between intracellular proline and extracellular proline or proline analogues.