GENETICS OF THE SERINE CYCLE IN METHYLOBACTERIUM-EXTORQUENS-AM1 - IDENTIFICATION, SEQUENCE, AND MUTATION OF 3 NEW GENES INVOLVED IN C-1 ASSIMILATION, ORF4, MTKA, AND MTKB

In a recent paper me reported the sequence of the beginning of a serine cycle gene cluster on the Methylobacterium extorquens AM1 chromosome, containing the genes encoding serine glyoxylate aminotransferase (sgaA), hydroxypyruvate reductase (hprA), and 5,10-methylenetetrahydrofolate dehydrogenase (mtdA) (L. V. Chistoserdova and M. E. Lidstrom J. Bacteriol. 176:1957-1968, 1994). Here we present the sequence of the adjacent downstream region containing three full and one partial open reading frames. The first of the full open reading frames (orf4) remains unidentified, while the other two (mtkA and mtkB) code for the two subunits of malate thiokinase, and the fourth, a partial open reading frame (ppcA), apparently encodes phosphoenolpyruvate carboxylase. Mutants containing insertion mutations in orf4, mtdA, and mtdB all were unable to grow on C-1 compounds, showing that these three newly identified genes are indispensable for the operation of the serine cycle. Mutants in orf4 mere also unable to grow on C-2 compounds, but growth was restored by glyoxylate, suggesting that orf4 might be required for the conversion of acetyl coenzyme A to glyoxylate.