Molecular Cloning, Sequencing, and Expression of a Novel Multidomain Mannanase Gene from Thermoanaerobacterium polysaccharolyticum

摘要

The manA gene of Thermoanaerobacterium polysaccharolyticum was cloned in Escherichia coli. The open reading frame of manA is composed of 3,291 bases and codes for a preprotein of 1,097 amino acids with an estimated molecular mass of 119,627 Da. The start codon is preceded by a strong putative ribosome binding site (TAAGGCGGTG) and a putative ?35 (TTCGC) and ?10 (TAAAAT) promoter sequence. The ManA of T. polysaccharolyticum is a modular protein. Sequence comparison and biochemical analyses demonstrate the presence of an N-terminal leader peptide, and three other domains in the following order: a putative mannanase-cellulase catalytic domain, cellulose binding domains 1 (CBD1) and CBD2, and a surface-layer-like protein region (SLH-1, SLH-2, and SLH-3). The CBD domains show no sequence homology to any cellulose binding domain yet reported, hence suggesting a novel CBD. The duplicated CBDs, which lack a disulfide bridge, exhibit 69% identity, and their deletion resulted in both failure to bind to cellulose and an apparent loss of carboxymethyl cellulase and mannanase activities. At the C-terminal region of the gene are three repeats of 59, 67, and 56 amino acids which are homologous to conserved sequences found in the S-layer-associated regions within the xylanases and cellulases of thermophilic members of theBacillus-Clostridium cluster. The ManA of T. polysaccharolyticum, besides being an extremely active enzyme, is the only mannanase gene cloned which shows this domain structure.