THE KAPPA-CARRAGEENASE OF THE MARINE BACTERIUM CYTOPHAGA-DROBACHIENSIS - STRUCTURAL AND PHYLOGENETIC-RELATIONSHIPS WITHIN FAMILY-16 GLYCOSIDE HYDROLASES

We report here cloning from the marine gliding bacterium Cytophaga dro bachiensis of kappa-carrageenase, a glycoside hydrolase involved in th e degradation of kappa-carrageenan. Structural features in the nucleot ide sequence are pointed out, including the presence of an octameric O mega sequence similar to the ribosome-binding sites of various eukaryo tes and prokaryotes. The cgkA gene codes for a protein of 545 aa, with a signal peptide of 35 aa and a 229-aa-long posttranslationaly proces sed C-terminal domain. The enzyme displays the overall folding and cat alytic domain characteristics of family 16 of glycoside hydrolases, wh ich comprises other beta-1,4-alpha-1,3-D/L-galactan hydrolases, beta-1 ,3-D-glucan hydrolases (laminarinases), beta-1,4-1,3-D-glucan hydrolas es (lichenases), and beta-1,4-D-xylaglucan endotransglycosylases. In o rder to address the origin and evolution of CgkA, a comprehensive phyl ogenetic tree of family 16 was built using parsimony analysis. Family- 16 glycoside hydrolases cluster according to their substrate specifici ty, regardless of their phylogenetic distribution over eubacteria and eukaryotes. Such a topology suggests that the general homology between laminarinases, agarases, kappa-carrageenases, lichenases, and xyloglu can endotransglycosylases has arisen through gene duplication, likely from an ancestral protein with laminarinase activity.