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J. Bacteriol., 02 1997, 643-649, Vol 179, No. 3
AR Reeves, GR Wang and AA Salyers
Results of earlier work had suggested that utilization of polysaccharides
by Bacteroides spp. did not proceed via breakdown by extracellular
polysaccharide-degrading enzymes. Rather, it appeared that the
polysaccharide was first bound to a putative outer membrane receptor
complex and then translocated into the periplasm, where the degradative
enzymes were located. In a recent article, we reported the cloning and
sequencing of susC, a gene from Bacteroides thetaiotaomicron that encoded a
115-kDa outer membrane protein. SusC protein proved to be essential for
utilization not only of starch but also of intermediate-sized
maltooligosaccharides (maltose to maltoheptaose). In this paper, we report
the sequencing of a 7-kbp region of the B. thetaiotaomicron chromosome that
lies immediately downstream of susC. We found four genes in this region
(susD, susE, susF, and susG). Transcription of these genes was maltose
inducible, and the genes appeared to be part of the same operon as susC.
Western blot (immunoblot) analysis using antisera raised against proteins
encoded by each of the four genes showed that all four were outer membrane
proteins. Protein database searches revealed that SusE had limited
similarity to a glucanohydrolase from Clostridium acetobutylicum and SusG
had high similarity to amylases from a variety of sources. SusD and SusF
had no significant similarity to any proteins in the databases. Results of
14C-starch binding assays suggested that SusD makes a major contribution to
binding. SusE and SusF also appear to contribute to binding but not to the
same extent as SusD. SusG is essential for growth on starch but appears to
contribute little to starch binding. Our results demonstrate that the
binding of starch to the B. thetaiotaomicron surface involves at least four
outer membrane proteins (SusC, SusD, SusE, and SusF), which may form a
surface receptor complex. The role of SusG in binding is still unclear.
Copyright © 1997, American Society for Microbiology
Characterization of four outer membrane proteins that play a role in utilization of starch by Bacteroides thetaiotaomicron
Department of Microbiology, University of Illinois, Urbana 61801, USA.
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