a 3 - phosphoglycerate dehydrogenase which exhibits a susceptibility to inhibition by serine which is reduced a as compared with that of an escherichia coli wild - type pgd and which possesses an amino acid sequence which differs from the amino acid sequence of the escherichia coli wild - type pgd in that an amino acid apart from glycine is present at position 249 or an amino acid apart from threonine is present at position 372 .