SUMMARY: has at least two permeases for L-proline. The gene of the gene cluster specifies the major proline permease, which is inducible by proline. Synthesis of the permease is subject to repression by ammonium at 37 ° but not at 25 °. A genetically unidentified minor proline permease(s) does not respond to proline induction or ammonium repression but is inhibited by ammonium. mutants are unable to utilize nitrogen sources other than ammonium because they lack a positive-acting regulatory product required for expression of ammonium-repressible activities. However, there are very few cases in which the lack of growth of mutants on a particular nitrogen source can be attributed to a reduction in the level of a particular enzyme activity or permease. Reduced expression of the permease can account for the inability of . This is demonstrated by the ability of -acting regulatory mutations designated , which derepress synthesis of the permease, to suppress mutations for proline utilization. The apparent ability of mutations to derepress synthesis of proline oxidase and l-pyrroline-5-carboxylate dehydrogenase is probably an indirect consequence of their ability to derepress synthesis of the permease, preventing inducer exclusion. There is presently no evidence that mutations directly affect expression of the , or genes, but this possibility has not been definitively eliminated.


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