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

Abstract

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.