CLONING, DNA-SEQUENCING, AND EXPRESSION OF THE GENE ENCODING CLOSTRIDIUM-THERMOCELLUM CELLULASE CELJ, THE LARGEST CATALYTIC COMPONENT OF THE CELLULOSOME

The Clostridium thermocellum F1 celJ gene, encoding endoglucanase J (C elJ), consists of an open reading frame (ORF) of 4,803 nucleotides and encodes a protein of 1,601 amino acids with a molecular weight of 178 ,055. The ORF was confirmed as cell by comparison with the N-terminal sequence of a truncated CelJ derivative. CelJ is a modular enzyme comp osed of N-terminal signal peptide and six domains in the following ord er: an S-layer homology domain, a domain of unknown function (UD-1), a subfamily El endoglucanase domain, a family J endoglucanase domain, a docking domain, and another domain of unknown function (UD-2). UD-1 h as no significant similarity to UD-2. CelJ hydrolyzed carboxymethylcel lulose and xylan, and xylanase activity was ascribed to the family J d omain. Antiserum raised against the truncated CelJ crossreacted with p roteins contained in the cellulosome of C. thermocellum F1. These resu lts strongly suggest that CelJ is equivalent to S-2, which was identif ied as the largest catalytic component in the cellulosome of C. thermo cellum YS. A second but incomplete ORF encoding an enzyme classified i n subfamily E2 endoglucanase, was located downstream of celJ.