MOLECULAR CHARACTERIZATION AND SEQUENCE OF A METHIONINE BIOSYNTHETIC LOCUS FROM PSEUDOMONAS-SYRINGAE

Two methionine biosynthetic genes in Pseudomonas syringae pv, syringae , metX and metW, were isolated, sequenced, and evaluated for their rol es in methionine biosynthesis and bacterial fitness on leaf surfaces, The metXW locus was isolated on a 1.8-kb DNA fragment that was require d for both methionine prototrophy and wildtype epiphytic fitness, Sequ ence analysis identified two consecutive open reading frames (ORFs), a nd in vitro transcription-translation experiments provided strong evid ence that the ORFs encode proteins with the predicted molecular masses of 39 and 22.5 kDa, The predicted amino acid sequence of MetX (39 kDa ) showed homology to several known and putative homoserine O-acetyltra nsferases. This enzyme is the first enzyme in the methionine biosynthe tic pathway of fungi, gram-negative bacteria of the genus Leptospira, and several grampositive bacterial genera, Both metX and metW were req uired for methionine biosynthesis, and transcription from both genes w as not repressed by methionine, MetW (22.5 kDa) did not show significa nt homology to any known protein, including prokaryotic and eukaryotic methionine biosynthetic enzymes, Several classes of methionine auxotr ophs, including metX and metW mutants, exhibit reduced fitness on leaf surfaces, indicating a requirement for methionine prototrophy in wild -type epiphytic fitness. This requirement is enhanced under environmen tally stressful conditions, suggesting a role for methionine prototrop hy in bacterial stress tolerance.