Summary: The systems which transport methionine in LT2 have been studied. Fourteen mutants, isolated by three different selection procedures, had similar growth characteristics and defects in the specific transport process showing a of 0.3 μM for L-methionine, and therefore lack the high-affinity, transport system. The sites of mutation in four of the mutants were shown by P1-mediated transduction to be linked (0.3 to 1.1 %) with a proline marker located at unit 7 on the chromosome. The high-affinity system was subject to both repression and transinhibition by methionine, and it may also be regulated by the and genes. There appeared to be at least two additional transport systems with relatively low affinities for methionine in the mutant strain, with apparent values for methionine of 24 μM and approximately 1.8 mM. The latter system, with a very low affinity for methionine, was inhibited by leucine. In addition, methionine inhibited leucine transport, suggesting that one of the low-affinity methionine transport systems may actually be a leucine transport system.


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