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J. Bacteriol., Apr 1995, 2178-2187, Vol 177, No. 8
N Kido, VI Torgov, T Sugiyama, K Uchiya, H Sugihara, T Komatsu, N Kato and K Jann
The rfb gene cluster of Escherichia coli O9 directs the synthesis of the
O9-specific polysaccharide which has the structure -->2-alpha-Man-
(1-->2)-alpha-Man-(1-->2)-alpha-Man-(1-->3)-alpha- Man-(1-->.
The E. coli O9 rfb cluster has been sequenced, and six genes, in addition
to the previously described rfbK and rfbM, were identified. They correspond
to six open reading frames (ORFs) encoding polypeptides of 261, 431, 708,
815, 381, and 274 amino acids. They are all transcribed in the counter
direction to those of the his operon. No gene was found between rfb and
his. A higher G+C content indicated that E. coli O9 rfb evolved
independently of the rfb clusters from other E. coli strains and from
Shigella and Salmonella spp. Deletion mutagenesis, in combination with
analysis of the in vitro synthesis of the O9 mannan in membranes isolated
from the mutants, showed that three genes (termed mtfA, -B, and -C,
encoding polypeptides of 815, 381, and 274 amino acids, respectively)
directed alpha-mannosyl transferases. MtfC (from ORF274), the first
mannosyl transferase, transfers a mannose to the endogenous acceptor. It
critically depended on a functional rfe gene (which directs the synthesis
of the endogenous acceptor) and initiates the growth of the polysaccharide
chain. MtfB (from ORF381) then transfers two mannoses into the 3 position
of the previous mannose, and MtfA (from ORF815) transfers three mannoses
into the 2 position. Further chain growth needs only the two transferases
MtfA and MtfB. Thus, there are fewer transferases needed than the number of
sugars in the repeating unit.(ABSTRACT TRUNCATED AT 250 WORDS)
Copyright © 1995, American Society for Microbiology
Expression of the O9 polysaccharide of Escherichia coli: sequencing of the E. coli O9 rfb gene cluster, characterization of mannosyl transferases, and evidence for an ATP-binding cassette transport system
Max-Planck-Institute fur Immunobiologie, Freiburg, Germany.
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