ABSTRACT
Abstract The cellulosomes of five cellulolytic Clostridia have been investigated. Cellulosomes are large molecular complexes which efficiently hydrolyze crystalline cellulose and where the catalytic subunits are anchored onto a non-enzymatic protein called scaffoldin. Many components of the various cellulosomes, produced as recombinant proteins in Escherichia coli, have been studied from a biochemical and structural point of view. Special attention was devoted to the cellulosome of a sixth Clostridium: Clostridium acetobutylicum. This bacterium is non-cellulolytic but recent sequencing of its genome revealed that this species harbours all the genes coding for cellulosomal components. In four of the six Clostridia, the genes encoding the major components of the cellulosome are organized in large clusters on the chromosome. The study of a spontaneous mutant of Clostridium cellulolyticum, affected in cellulolysis, provided interesting information on the regulation of the genes belonging to these clusters. Previously established genetic tools, now available for the two mesophilic Clostridia, C. cellulolyticum and C. acetobutylicum, will allow new approaches for studying the cellulosomes “w clostridio”
1. Introduction Cellulose is the most important biopolymer on earth and constitutes the major component of plant cell wall where it is associated with other structural polymers: hemicellulose, pectin and lignin. In nature, cellulose is degraded by a wide variety of microorganisms and this degradation plays a fundamental role in carbon recycling on the planet. The major part of the cellulosic material is degraded in aerobiosis by white-rot fungi, soft-rot fungi and aerobic bacteria. About 10% of the cellulosic material is degraded in anaerobiosis essentially by bacteria. In anaerobic biotopes, these bacteria are often accompanied by non-cellulolytic bacteria, such as
acetogens and methanogens; these consortia allow the entire mineralization of the biomass to methane and carbon dioxide. Despite numerous studies, attempts to enhance the rate of biomass fermentation for chemical or biofuel production had only limited success as hydrolysis of its polymeric components, especially cellulose, confers limits upon the rate of the entire process.
