GENETICS OF SERINE PATHWAY ENZYMES IN METHYLOBACTERIUM-EXTORQUENS AM1- PHOSPHOENOLPYRUVATE CARBOXYLASE AND MALYL COENZYME-A LYASE

Methylobacterium extorquens AM1 is a facultative methylotrophic bacter ium that uses the serine pathway for formaldehyde incorporation as its assimilation pathway during growth on one-carbon compounds. A DNA reg ion from M. extorquens AM1 previously shown to contain genes for the s erine pathway enzymes malyl coenzyme A (CoA) lyase and hydroxypyruvate reductase has been characterized in more detail. Insertion mutagenesi s revealed an additional region required for growth on one-carbon comp ounds, and all of the insertion mutants in this region lacked activity for another serine pathway enzyme, the acetyl-CoA-independent phospho enolpyruvate (PEP) carboxylase. Expression analysis with Escherichia c oli of DNA fragments that included the malyl-CoA lyase and PEP carboxy lase regions identified five polypeptides, all transcribed in the same direction. Three of these polypeptides were expressed from the region necessary for the acetyl-CoA-independent PEP carboxylase, one was exp ressed from the region containing the malyl-CoA lyase gene, and the fi fth was expressed from a region immediately downstream from the gene e ncoding hydroxypyruvate reductase. All six genes are transcribed in th e same direction, but the transposon insertion data suggest that they are not all cotranscribed.