Cellulose, a
linear polysaccharide, is the most abundant carbon source on earth, yet its
recalcitrant nature renders it unavailable for bio-degradation. Among several cellulolytic
microorganisms is Ruminococcus flavefaciens FD-1 that is prominent in
the rumen of herbivores. Recently its full genome was sequenced, revealing an
elaborate cellulosome system with a unique set of small scaffoldins and as many
as 222 dockerin-bearing proteins. Bio-informatics analysis of these dockerin
sequences resulted in their classification into six major groups and eleven
subgroups. Since these modules are directly engaged in constructing cellulosome
architecture, it was important to search for their cohesin partners and test if
the observed sequence variation translates into functional discrepancies. In
this study, a network of 462 possible interactions between 14 chosen cohesins
and 33 dockerins, chosen to represent the distinct groups, were experimentally
put to test, while improving a previously established cellulose-based
microarray method for work with crude cell extracts. A total of 41 previously
unknown positive interactions were found, and a clear preference of cohesin
recognition by the six major dockerin groups was observed. The second-order
classification was found to be functionally redundant. Three novel putative
adaptor scaffoldins emerged: ScaJ, ScaM and ScaO. The novel pairs found herein
provide a snapshot into the intricate cellulosome organization of R.
flavefaciens FD-1, and an updated model is proposed. This knowledge may be applicable
in future conversion of lignocellulosic biomass into fermentable monosaccharides
for biofuel production and cellulosic waste management.
