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 fragment which was isolated from a partial Sau3A library in the cloning vector pUC19 using 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal) as the selection. The enzyme produced by the clone has a specificity for beta 1-3- > beta 1-4-linked galactose. The nucleotide sequence of 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 eukaryotic beta-galactosidases from animals, plants and fungi. The strongest similarity was with the beta-galactosidases found in the human and mouse lysosomes (42 and 41% identity, respectively). Alignment of the X.manihotis and eukaryotic beta-galactosidase sequences revealed seven highly conserved domains common to each protein. Additionally, Domain 1 in X.manihotis showed similarity to regions within catalytic domains from seven xylanases and cellulases belonging to family 10 of glucosyl hydrolases. A region spanning Domain 2 showed similarity to the catalytic domain of endo beta 1-3 glucanases from tobacco and barley.