Structure of the Clostridium thermocellum gene licB and the encoded β-1,3-1,4-glucanase

A catalytic region homologous to Bacillus lichenases joined to the reiterated domain of clostridial cellulases

The nucleotide sequence of the Clostridium thermocellum gene licB, coding for a thermoactive β-1,3-1,4-glucanase, has been determined. The gene is located downstream, but in opposite orientation to the β-glucosidase gene bglA. A coding region of 1002 bp is flanked by canonical promoter and transcription terminator sequences. The primary translation product of the licB gene has a predicted molecular mass of 37896 Da. The protein sequence can be divided into several discrete segments: an N-terminal signal peptide, a catalytic region, a segment rich in Pro and Thr residues and a C-terminal reiterated domain. The catalytic region shows close similarity to lichenases of bacilli (52 – 58% identity) and Fibrobacter succinogenes (35% identity), but is unrelated to barley β-1,3-1,4-glucanases. It consists of two domains, which in the case of the F. succinogenes lichenase are arranged in reversed order to that of C. thermocellum and Bacillus lichenases. The C-terminal reiterated domain of C. thermocellum lichenase is homologous to the duplicated non-catalytic domain of endo-β-1,4-glucanases and xylanase Z from the same organism. This domain is considered a characteristic feature of clostridial cellulases organized as multienzyme complex (cellulosome). The β-1,3-1,4-glucanase encoded by the licB gene might therefore be an additional enzyme component of the C. thermocellum cellulosome.