W. M. Keck Laboratories, California Institute of Technology, Pasadena 91125.
The gene encoding the serine cycle hydroxypyruvate reductase of Methylobacterium extorquens AM1 was isolated by using a synthetic oligonucleotide with a sequence based on a known N-terminal amino acid sequence. The cloned gene was inactivated by insertion of a kanamycin resistance gene, and recombination of this insertion derivative with the wild-type gene produced a serine cycle hydroxypyruvate reductase null mutant. This mutant had lost its ability to grow on C-1 compounds but retained the ability to grow on C-2 compounds, showing that the hydroxypyruvate reductase operating in the serine cycle is not involved in the conversion of acetyl coenzyme A to glycine as previously proposed. A second hydroxypyruvate-reducing enzyme with a low level of activity was found in M. extorquens AM1; this enzyme was able to interconvert glyoxylate and glycollate. The gene encoding hydroxypyruvate reductase was shown to be located about 3 kb upstream of two other serine cycles genes encoding phosphoenolpyruvate carboxylase and malyl coenzyme A lyase.