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J. Bacteriol., 10 1995, 5539-5546, Vol 177, No. 19
CL Marolda and MA Valvano
The O-repeating unit of the Escherichia coli O7-specific lipopolysaccharide
is made of galactose, mannose, rhamnose, 4-acetamido- 4,6-dideoxyglucose,
and N-acetyglucosamine. We have recently characterized the genes involved
in the biosynthesis of the sugar precursor GDP-mannose occurring in the E.
coli O7:K1 strain VW187 (C. L. Marolda and M. A. Valvano, J. Bacteriol.
175:148-158, 1993). In the present study, we identified and sequenced the
rfbBDAC genes encoding the enzymes for the biosynthesis of another
precursor, dTDP-rhamnose. These genes are localized on the upstream end of
the rfbEcO7 region, and they are strongly conserved compared with similar
genes found in various enteric and nonenteric bacteria. Upstream of rfbB we
identified a DNA segment containing the rfb promoter and a highly conserved
untranslated leader sequence also present in the promoter regions of other
surface polysaccharide gene clusters. Also, we have determined that rfbB
and rfbA have homologs, rffG (o355) and rffH (o292), respectively, located
on the rff cluster, which is involved in the synthesis of enterobacterial
common antigen. We provide biochemical evidence that rffG and rffH encode
dTDP-glucose dehydratase and glucose- 1-phosphate thymidylyltransferase
activities, respectively, and we also show that rffG complemented the rfbB
defect in the O7+ cosmid pJHCV32. We also demonstrate that rffG is distinct
from rffE and map the rffE gene to the second gene of the rff cluster.
Copyright © 1995, American Society for Microbiology
Genetic analysis of the dTDP-rhamnose biosynthesis region of the Escherichia coli VW187 (O7:K1) rfb gene cluster: identification of functional homologs of rfbB and rfbA in the rff cluster and correct location of the rffE gene
Department of Microbiology and Immunology, University of Western Ontario, London, Canada.
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