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Journal of Bacteriology, September 1998, p. 4497-4507, Vol. 180, No. 17
Department of Plant and Microbial Biology,
University of California, Berkeley, Berkeley, California 94720
Received 7 January 1998/Accepted 25 June 1998
Two methionine biosynthetic genes in Pseudomonas
syringae pv. syringae, metX and
metW, were isolated, sequenced, and evaluated for
their roles in methionine biosynthesis and bacterial fitness on leaf
surfaces. The metXW locus was isolated on a 1.8-kb
DNA fragment that was required for both methionine prototrophy
and wild-type epiphytic fitness. Sequence analysis
identified two consecutive open reading frames (ORFs), and in vitro
transcription-translation experiments provided strong evidence 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-acetyltransferases. This enzyme is the first enzyme in
the methionine biosynthetic pathway of fungi, gram-negative
bacteria of the genus Leptospira, and several gram-positive bacterial genera. Both metX and
metW were required for methionine biosynthesis, and
transcription from both genes was not repressed by methionine. MetW
(22.5 kDa) did not show significant homology to any known protein,
including prokaryotic and eukaryotic methionine biosynthetic enzymes.
Several classes of methionine auxotrophs, 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 environmentally stressful conditions,
suggesting a role for methionine prototrophy in bacterial stress
tolerance.
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Molecular Characterization and Sequence of a
Methionine Biosynthetic Locus from Pseudomonas
syringae

and
*
Corresponding author. Mailing address: Department of
Plant and Microbial Biology, University of California, 111 Koshland
Hall, Berkeley, CA 94720-3102. Phone: (510) 642-4174. Fax: (510)
643-5098. E-mail: icelab{at}socrates.berkeley.edu.
Present address: Biology & Biotechnology Research Program, Lawrence
Livermore National Laboratory, Livermore, CA 94550.
Present address: Department of Microbiology, Immunology and
Preventive Medicine, Iowa State University, Ames, IA 50011.
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