Molecular diversity of thermophilic cellulolytic and hemicellulolytic bacteria

Abstract

Many thermophilic bacteria belong to groups with deep phylogenetic lineages and ancestral forms were established before the occurrence of eucaryotes that produced cellulose and hemicellulose. Thus they may have acquired their beta-glycanase genes from more recent mesophilic bacteria. Most research has focussed on extremely thermophilic eubacteria growing above 65 degrees C under anaerobic conditions. Only recently have aerobic cellulolytic thermophiles been described from widely separated lineages (for example, Rhodothermus marinus, Caldibacillus cellulovorans). Many thermophilic bacteria produce cellulases and xylanases that have novel structures, with additional protein domains not identified with their catalytic activity. Many of these enzymes are multifunctional and code for more than one catalytic activity. This type of enzyme structure was first identified in the extreme thermophile Caldicellulosiruptor caccharolyticus. There is a general relatedness evident between catalytic domains, cellulose binding domains and other ancillary domains, which suggests that there may have been significant lateral gene transfer in the evolution of these microorganisms. Detailed molecular studies show that there is variation in the sequences of these related but not identical genes from taxonomically widely-separated organisms.