INTRONLESS CELB FROM THE ANAEROBIC FUNGUS NEOCALLIMASTIX PATRICIARUM ENCODES A MODULAR FAMILY A-ENDOGLUCANASE

The cDNA designated celB from the anaerobic rumen fungus Neocallimasti x patriciarum contained a single open reading frame of 1422 bp coding for a protein (CelB) of M(r) 53070. CelB expressed by Escherichia coli harbouring the full-length gene hydrolysed carboxymethylcellulose in the manner of an endoglucanase, but was most active against barley bet a-glucan. It also released reducing sugar from xylan and lichenan, but was inactive against crystalline cellulose, laminarin, mannan, galact an and arabinan. The rate of hydrolysis of cellulo-oligosaccharides by CelB increased with increasing chain length from cellotriose to cello pentaose. The predicted structure of CelB contained features indicativ e of modular structure. The first 360 residues of CelB constituted a f ully functional catalytic domain that was homologous with bacterial en doglucanases belonging to cellulase family A, including five which ori ginate from three different species of anaerobic rumen bacteria. Downs tream from this domain, and linked to it by a serine/threonine-rich hi nge, was a non-catalytic domain containing short tandem repeats, homol ogous to the C-terminal repeats contained in xylanase A from the same anaerobic fungus. Unlike previous fungal cellulases, genomic celB was devoid of introns. This lack of introns and the homology of its encode d product with rumen bacterial endoglucanases suggest that acquisition of celB by the fungus may at some stage have involved horizontal gene transfer from a prokaryote to N. patriciarum.