Abstract
Extremely thermophilic bacteria of the genus Caldicellulosiruptor utilize carbohydrate components of plant cell walls, including
cellulose and hemicellulose, facilitated by a diverse set of glycoside hydrolases (GHs). From a biofuel perspective, this capability
is crucial for deconstruction of plant biomass into fermentable sugars. While all species from the genus grow on xylan and acidpretreated
switchgrass, growth on crystalline cellulose is variable. The basis for this variability was examined using microbiological,
genomic, and proteomic analyses of eight globally diverse Caldicellulosiruptor species. The open Caldicellulosiruptor pangenome
(4,009 open reading frames [ORFs]) encodes 106 GHs, representing 43 GH families, but only 26 GHs from 17 families
are included in the core (noncellulosic) genome (1,543 ORFs). Differentiating the strongly cellulolytic Caldicellulosiruptor species
from the others is a specific genomic locus that encodes multidomain cellulases from GH families 9 and 48, which are associated
with cellulose-binding modules. This locus also encodes a novel adhesin associated with type IV pili, which was identified in
the exoproteome bound to crystalline cellulose. Taking into account the core genomes, pangenomes, and individual genomes,
the ancestral Caldicellulosiruptor was likely cellulolytic and evolved, in some cases, into species that lost the ability to degrade
crystalline cellulose while maintaining the capacity to hydrolyze amorphous cellulose and hemicellulose.
