Cloning and sequence analysis of the Escherichia coli metH gene encoding cobalamin-dependent methionine synthase and isolation of a tryptic fragment containing the cobalamin-binding domain

A gene encoding cobalamin-dependent methionine synthase (EC 2.1.1.13) has been isolated from a plasmid library of Escherichia coli K-12 DNA by complementation to methionine prototrophy in an E. coli strain lacking both cobalamin-dependent and -independent methionine synthase activities (RK4536:metE, metHH). Maxicell expression of a series of plasmids containing deletions in the metH structural gene was employed to map the position and orientation of the gene on the cloned DNA fragment. A 6.3-kilobase EcoRI-SalI fragment containing the gene was cloned into the sequencing vector pGEM3B for double-stranded DNA sequencing; the MetH coding region consists of 3372 nucleotides. The enzyme was purified from an overproducing strain of E. coli harboring the recombinant plasmid, in which the level of methionine synthase was elevated 30- to 40-fold over wild-type E. coli. Recombinant enzyme is a protein of 123,640 molecular weight and has a turnover number of 1,450 min-1 in the standard assay. These values are to be compared with previously reported values of 133,000 for the molecular weight and 1,240-1,560 min-1 for the turnover number of the homogenous enzyme purified from a wild-type strain of E. coli B (Frasca, V., Banerjee, R. V., Dunham, W. R., Sands, R. H., and Matthews, R. G. (1988) Biochemistry 27, 8458-8465). Limited proteolysis of the native enzyme with trypsin resulted in loss of enzyme activity but retention of bound cobalamin on a peptide fragment of 28,000 molecular weight. This fragment has been shown to extend from residue 643 to residue 900 of the 1124-residue deduced amino acid sequence.