Evolution in biosynthetic pathways: Two enzymes catalyzing consecutive steps in methionine biosynthesis originate from a common ancestor and possess a similar regulatory region (homology/,B-cystathionase/cystathionine y synthase/sequence/operator)

The metC gene of Escherichia coli K-12 was cloned and the nucleotide sequence of the 'metC gene and its flanking regions was determined. The translation initiation codon was identified by sequencing the NH2-terminal part of fi-cystathionase, the MetC gene product. The metC gene (1185 nucleotides) encodes a protein having 395 amino acid residues. The 5' noncoding region was found to contain a "Met box"; homologous to sequences suggestive of operator .structures upstream from other methionine genes that are controlled by the product of the pleiotropic regulatory metJ gene. The deduced amino acid sequence of p-cystathionase showed ex- tensive homology with that of the MetB protein (cystathionine y-synthase) that catalyzes the preceding step in methionine biosynthesis. The homology strongly suggests that the struc- tural genes for the MetB and MetC proteins evolved from a common ancestral gene.