Transport and utilization of D-methionine and other methionine sources in Escherichia coli

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Abstract
The transport and utilization of D-methionine was investigated in several strains of E. coli K-12. Wild-type cells exhibit a single transport system with a Km of 1.16 .mu.M. This activity exhibits a specificity similar to that of the uptake of L-methionine. The activity toward the D-isomer and the high-affinity uptake of L-methionine are lost in strains mutant in metD, along with the ability to utilize D-methionine as methionine source. Both activities respond identically to gene dosage of metD and are both restored in revertants or transductants. Although L-methionine is a potent inhibitor of D-methionine uptake, D-methionine has little or no effect on the uptake of the L-isomer. No mutants altered in the uptake of only 1 of the 2 isomers were found in a screening. Regulation of both activities was similar in their response to the internal methionine pool, and evidence indicates partial repressive control of these activities. The evidence is most consistent with the role of the metD product as a common step for 2 methionine-specific uptake systems, but other gene products may represent the initial substrate binding sites. This system also may be involved in the uptake of N-acetyl methionine and methionine sulfoxide and methionine sulfoximine. The uptake of the keto analog of methionine, .alpha.-keto-.gamma.-methiol butyrate, may be mediated by a separate system specific for .alpha.-keto straight-chain acids 5-6 C units in length.