Control of methionine biosynthesis in K12 was reinvestigated by using methionine-analogue-resistant mutants. Norleucine (NL) and α-methylmethionine (MM) were found to inhibit methionine biosynthesis directly whereas ethionine (Et) competitively inhibited methionine utilization. Adenosylation of Et to generate -adenosylethionine (AdoEt) by cell-free enzyme from K12 was demonstrated. Tolerance of increasing concentrations of NL by K12 mutants is expressed serially as phenotypes NL, NLEt, NLMM and finally NLEtMM. All spontaneous NL mutants had a mutation, whereas NTG-induced mutants had mutations in both the and genes. The kinetics of methionine adenosylation by the K12 cell-free enzyme were found to be similar to those reported for the yeast enzyme, showing the typical lag phase at low methionine concentration and disappearance of this phase when AdoMet was included in the incubation mixture. NL extended the lag phase, and lowered the rate of subsequent methionine adenosylation, but did not affect the shortening of the lag phase of adenosylation by AdoMet.


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