The third chitinase gene (chiC) of Serratia marcescens 2170 and the relationship of its product to other bacterial chitinases

The third chitinase gene (chiC) of Serratia marcescens 2170, specifying chi tinases C1 and C2, was identified. Chitinase C1 lacks a signal sequence and consists of a catalytic domain belonging to glycoside hydrolase family 18, a fibronectin type III-like domain (Fn3 domain) and a C-terminal chitin-bi nding domain (ChBD). Chitinase C2 corresponds to the catalytic domain of C1 and is probably generated by proteolytic removal of the Fn3 and ChBDs. The loss of the C-terminal portion reduced the hydrolytic activity towards pow dered chitin and regenerated chitin, but not towards colloidal chitin and g lycol chitin, illustrating the importance of the ChBD for the efficient hyd rolysis of crystalline chitin. Phylogenetic analysis showed that bacterial family 18 chitinases can be clustered in three subfamilies which have diver ged at an early stage of bacterial chitinase evolution. Ser. marcescens chi tinase C1 is found in one subfamily, whereas chitinases A and B of the same bacterium belong to another subfamily. Chitinase C1 is the only Ser. marce scens chitinase that has an Fn3 domain. The presence of multiple, divergent , chitinases in a single chitinolytic bacterium is perhaps necessary for ef ficient synergistic degradation of chitin.