Comparative analysis of the Geobacillus hemicellulose utilization locus reveals a highly variable target for improved hemicellulolysis

Abstract

Background: Members of the thermophilic genus Geobacillus can grow at high temperatures and produce a battery of thermostable hemicellulose hydrolytic enzymes, making them ideal candidates for the bioconversion of biomass to value-added products. To date the molecular determinants for hemicellulose degradation and utilization have only been identified and partially characterized in one strain, namely Geobacillus stearothermophilus T-6, where they are clustered in a single genetic locus. Results: Using the G. stearothermophilus T-6 hemicellulose utilization locus as genetic marker, orthologous hemicellulose utilization (HUS) loci were identified in the complete and partial genomes of 17/24 Geobacillus strains. These HUS loci are localized on a common genomic island. Comparative analyses of these loci revealed extensive variability among the Geobacillus hemicellulose utilization systems, with only seven out of 41-68 proteins encoded on these loci conserved among the HUS+ strains. This translates into extensive differences in the hydrolytic enzymes, transport systems and metabolic pathways employed by Geobacillus spp. to degrade and utilize hemicellulose polymers. Conclusions: The genetic variability among the Geobacillus HUS loci implies that they have variable capacities to degrade hemicellulose polymers, or that they may degrade distinct polymers, as are found in different plant species and tissues. The data from this study can serve as a basis for the genetic engineering of a Geobacillus strain(s) with an improved capacity to degrade and utilize hemicellulose.