Molecular diversity of thermophilic cellulolytic and hemicellulolytic bacteria

Many thermophilic bacteria belong to groups with deep phylogenetic lineages and ancestral forms were established before the occurrence of eucaryotes t hat produced cellulose and hemicellulose. Thus they may have acquired their beta-glycanase genes from more recent mesophilic bacteria. Most research h as focussed on extremely thermophilic eubacteria growing above 65 degrees C under anaerobic conditions. Only recently have aerobic cellulolytic thermo philes been described from widely separated lineages (for example, Rhodothe rmus marinus, Caldibacillus cellulovorans). Many thermophilic bacteria prod uce cellulases and xylanases that have novel structures, with additional pr otein domains not identified with their catalytic activity. Many of these e nzymes are multifunctional and code for more than one catalytic activity. T his type of enzyme structure was first identified in the extreme thermophil e Caldicellulosiruptor caccharolyticus. There is a general relatedness evid ent between catalytic domains, cellulose binding domains and other ancillar y domains, which suggests that there may have been significant lateral gene transfer in the evolution of these microorganisms. Detailed molecular stud ies show that there is variation in the sequences of these related but not identical genes from taxonomically widely-separated organisms. (C) 1999 Fed eration of European Microbiological Societies. Published by Elsevier Scienc e B.V. All rights reserved.