A NOVEL BETA-GALACTOSIDASE GENE ISOLATED FROM THE BACTERIUM XANTHOMONAS-MANIHOTIS EXHIBITS STRONG HOMOLOGY TO SEVERAL EUKARYOTIC BETA-GALACTOSIDASES

The gene encoding a beta-galactosidase from Xanthomonas manihotis was cloned into Escherichia coli, The gene resides on a 2.4 kb DNA fragmen t which was isolated from a partial Sau3A library in the cloning vecto r pUC19 using -bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-ga l) as the selection. The enzyme produced by the clone has a specificit y for beta 1-3- > beta 1-4-linked galactose, The nucleotide sequence o f the gene was determined, The deduced protein sequence contained 597 amino acids yielding a monomeric molecular mass of 66 kDa, The cloned beta-galactosidase showed no similarity to any known prokaryotic beta- galactosidase. However, extensive similarity was observed with eukaryo tic beta-galactosidases from animals, plants and fungi, The strongest similarity was with the beta-galactosidases found in the human and mou se lysosomes (42 and 41% identity, respectively), Alignment of the X.m anihotis and eukaryotic beta-galactosidase sequences revealed seven hi ghly conserved domains common to each protein, Additionally, Domain 1 in X.manihotis showed similarity to regions within catalytic domains f rom seven xylanases and cellulases belonging to family 10 of glucosyl hydrolases. A region spanning Domain 2 showed similarity to the cataly tic domain of endo beta 1-3 glucanases from tobacco and barley.