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An
obligatory step in cellulose degradation by anaerobic
bacteria is the adhesion of the bacterium to
the polysaccharide. In many anaerobic bacteria
the adhesion protein, and the enzymes required
for polysaccharide hydrolysis, are organized
into a complex and interesting structure called
the cellulosome. The genetics and biochemical
characteristics of cellulosomes are quite diverse,
but very little is known about the events underpinning
the synthesis and assembly of these interesting
complexes. My laboratory is currently examining
the production and characteristics of these complexes
in a variety of anaerobic cellulolytic and non-cellulolytic
bacteria. My lab is also interested in the adhesion
of bacteria to plant surfaces, and we have identified
a novel form of cellulose-binding protein, produced
by the gram-positive, cellulose-degrading anaerobe Ruminococcus
albus. This protein is a member of the Pil-protein
family, and is very similar to the type 4 fimbrial
proteins of Gram-negative, pathogenic bacteria.
These studies have provided new insights into
the adhesion of bacteria to plant surfaces, and
call attention to the likely existence of genetically
analogous adhesion determinants in both pathogenic
and non-pathogenic bacteria. A number of cellulose-degrading
microorganisms are now the subject of genome
sequencing projects, which will facilitate a
more global analysis of the genetics and molecular
biology controlling this key process in the cellulose
degradation.
My laboratory is also studying the microbial interactions
that support optimal rates and extent of cellulose
degradation. One example is the requirement by Ruminococcus
albus for micromolar amounts of phenylacetic
and phenylpropionic acids, which stimulate the
formation of cellulosome-like complexes and maximal
rates of cellulose degradation. Other members of
the microbial community produce these phenyl-substituted
fatty acids, and my laboratory is currently investigating
how R. albus responds to these compounds,
by using techniques such as differential display
RT-PCR and proteomics. A second example is the
synergistic association between cellulose-degrading
eubacteria and methanogenic archaea, via the process
known as interspecies hydrogen transfer. Many cellulose-degrading
bacteria will produce hydrogen during fermentation,
which permits the bacterium to produce additional
ATP by substrate-level phosphorylation, and affords
faster rates of growth and cellulose degradation.
However, the cellulose-degrading bacteria are dependent
on the consumption of hydrogen by autotrophic methanogens
to maintain this energetically favorable pathway
of fermentation. Now that the genomes of several
methanogens and cellulose-degrading bacteria have
been completely sequenced, it will be possible
to investigate this synergistic association more
completely. An ultimate goal for our research is
to use genomics and related methods to understand
the molecular details of these microbial interactions
and improve the bioconversion of cellulosic wastes
into useful products such as ethanol.
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Morrison, M. and Miron, J. 2000. MiniReview: Adhesion
to cellulose by Ruminococcus albus: a combination
of cellulosomes and Pil-proteins? FEMS Microbiol.
Letts. 185: 109-115
Heng, N.C.K., Bateup, J.M.,
Loach, D.M., Wu, X., Jenkinson, H.F., Morrison,
M. and Tannock, G.W.
1999. The influence of different functional elements
of plasmid pGT232 on the maintenance of recombinant
plasmids in Lactobacillus reuteri populations
in vitro and in vivo. Appl. Environ. Microbiol.
65: 5378-5385.
Larson, M.A. and Morrison, M. 1999.
Application of the differential display RT-PCR
technique to
examine conditional gene expression in Ruminococcus
albus. In: Bell, C.R. et al. (Eds.) Microbial
Biosystems: New Frontiers. Proc. 8th Int. Symp.
Microb. Ecol
Pegden, R. S., Larson, M.A., Grant, R.J., and
Morrison, M. 1998. Adherence of the gram-positive
bacterium Ruminococcus albus to cellulose,
and identification of a novel form of cellulose-binding
protein which belongs to
the Pil-family of proteins. J. Bacteriol. 180: 5921-5927.
White, B.A., Cann, I.K.O., Mackie, R.I., and Morrison, M. 1997. Cellulase
and xylanase genes from ruminal bacteria. Domain analysis suggests a non-cellulosome
model for organization of the cellulase complex. pp. 69-80. In: Onodera,
R.
et al. (Eds.) "Rumen Microbes and Digestive Physiology of Ruminants " Japan
Scientific Societies Press, Tokyo, Japan.
Mark
Morrison and the MAPLE research team located
in the Department of Animal Sciences |
