A second Escherichia coli gene with similarity to gapA.

An open reading frame has been found downstream of the ald gene at 31 min in the Escherichia coli chromosome and has been designated gapC because of its high similarity with gapA (min 39, encoding glyceraldehyde-3-phosphate dehydrogenase), and with gapB (min 62, a gene with high similarity to gapA, encoding erythrose-4-phosphate dehydrogenase). The gapC gene (min 31) encodes a polypeptide of 204 amino acids, 126 residues shorter than glyceraldehyde-3-phosphate dehydrogenase. In this 204-codon open reading frame several amino acids important for catalysis are conserved. However, the cofactor binding site is lost. The results illustrate a case of a gene, encoding a glycolytic enzyme, for which at least three copies maintaining a certain degree of similarity are apparent in the E. coli genome. It seems likely that the genes encode products with different cellular functions. The origin of these three copies of the gap gene by horizontal transfer or by duplication of an ancestral gene is discussed.