CELA, ANOTHER GENE CODING FOR A MULTIDOMAIN CELLULASE FROM THE EXTREME THERMOPHILE CALDOCELLUM-SACCHAROLYTICUM

Caldocellum saccharolyticum is an extremely thermophilic anaerobic bac terium capable of growth on cellulose and hemicellulose as sole carbon sources. Cellulase and hemicellulase genes have been found clustered together on its genome. The gene for one of the cellulases (celA) was isolated on a lambda genomic library clone, sequenced and found to com prise a large open-reading frame of 5253 base pairs that could be tran slated into a peptide of 1751 amino acids. To date, it is the largest cellulase gene sequenced. The translated product is a multidomain stru cture composed of two catalytic domains and two cellulose-binding doma ins linked by proline-threonine-rich regions (PT linkers). The N-termi nal domain of celA encodes for an endoglucanase activity on carboxymet hylcellulose, consistent with its high homology to the sequences of se veral other endo-1,4-beta-D-glucanases. The carboxylterminal domain sh ows sequence homology with a cellulase from Clostridium thermocellum ( CelS), which is known to act synergistically with a second component t o hydrolyze crystalline cellulose. In the absence of a Caldocellum hom ologue for this second protein, we can detect no activity from this do main.