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J. Bacteriol., 12 1996, 7173-7179, Vol 178, No. 24
JN D'Elia and AA Salyers
Bacteroides thetaiotaomicron, a gram-negative colonic anaerobe, can utilize
three forms of starch: amylose, amylopectin, and pullulan. Previously, a
neopullulanase, a pullulanase, and an alpha-glucosidase from B.
thetaiotaomicron had been purified and characterized biochemically. The
neopullulanase and alpha-glucosidase appeared to be the main enzymes
involved in the breakdown of starch, because they were responsible for most
of the starch-degrading activity detected in B. thetaiotaomicron cell
extracts. To determine the importance of these enzymes in the starch
utilization pathway, we cloned the genes encoding the neopullulanase and
alpha-glucosidase. The gene encoding the neopullulanase (susA) was located
upstream of the gene encoding the alpha-glucosidase (susB). Both genes were
closely linked to another starch utilization gene, susC, which encodes a
115-kDa outer membrane protein that is essential for growth on starch. The
gene encoding the pullulanase, pulI, was not located in this region in the
chromosome. Disruption of the neopullulanase gene, susA, reduced the rate
of growth on starch by about 30%. Elimination of susA in this strain
allowed us to detect a low residual level of enzyme activity, which was
localized to the membrane fraction. Previously, we had shown that a
disruption in the pulI gene did not affect the rate of growth on pullulan.
We have now shown that a double mutant, with a disruption in susA and in
the pullulanase gene, pulI, was also able to grow on pullulan. Thus, there
is at least one other starch-degrading enzyme besides the neopullulanase
and the pullulanase. Disruption of the alpha-glucosidase gene, susB,
reduced the rate of growth on starch only slightly. No residual
alpha-glucosidase activity was detectable in extracts from this strain.
Since this strain could still grow on maltose, maltotriose, and starch,
there must be at least one other enzyme capable of degrading the small
oligomers produced by the starch- degrading enzymes. Our results show that
the starch utilization system of B. thetaiotaomicron is quite complex and
contains a number of apparently redundant degradative enzymes.
Copyright © 1996, American Society for Microbiology
Contribution of a neopullulanase, a pullulanase, and an alpha- glucosidase to growth of Bacteroides thetaiotaomicron on starch
Department of Microbiology, University of Illinois, Urbana 61801, USA.
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