Control of methionine biosynthesis in Escherichia coli K12 was reinvestigated by using methionine-analogueresistant mutants. Norleucine (NL) and a-methylmethionine (MM) were found to inhibit methionine biosynthesis directly whereas ethionine (Et) competitively inhibited methionine utilization. Adenosylation of Et to generate Sadenosylethionine (AdoEt) by cell-free enzyme from E. coli K12 was demonstrated. Tolerance of increasing concentrations of NL by E. coli K12 mutants is expressed serially as phenotypes NLR, NLR EtR, NLR MMR and finally NLR EtR MMR. All spontaneous NLR mutants had a metK mutation, whereas NTG-induced mutants had mutations in both the metX and met.! genes. The kinetics of methionine adenosylation by the E. coli 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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