Bacillus cellulyticus K-12 Avicelase (Avicelase I; EC 3.2.1.4) gene (ace A) has been cloned in Escherichia coli by using the vector pT7T3U19 and HindIII-HindIII libraries of the chromosomal inserts. The libraries were Screened for the expression of avicelase by monitoring the immunoreaction of the antiavicelase (immunoscreening), Positive clones (Ac-3, Ac-5, and Ac-7) contained the identical 3.5-kb HindIII fragment as determined by restriction mapping and Southern hybridization, and expressed avicelase efficiently and constitutively using its own promoter in the heterologous host. From the immunoblotting analysis, a polypeptide that showed a carboxymethylcellulase (CMCase) activity with an M-r of 64,000 was detected. The recombinant endo 1,4-beta-(D)-glucanase I was purified to homogeneity from an intracellular fraction of E. coli by DEAE-Toyopearl M650, Phenyl Toyoperal M650, and TSK gel HW50S chromatography. The enzyme had a monomeric structure, its relative molecular mass being 65 kDa by gel filtration and 64 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The pI was 5.3 and the optimal pH was 4.6, and the enzyme was stable at PH 4.0-10.5. The enzyme had a temperature optimum of 50 degrees C and was stable at 55 degrees C for 48 h, and retained approx 20% of its activity after 30 min at 70 degrees C. It showed high activity toward carboxymethylcellulose (CMC) as well as p-nitrophenyl-beta-D-cellobioside, 4-methylumbelliferyl cellobioside, Avicel, filter paper, and some cellooligosaccharides. K-m values for CMC and Avicel were 7.6 and 85.2 mg/mL, respectively, whereas V-max values were 201 and 9.2 mu mol.min(-1).mg(-1), respectively. Cellotetraose (G4) was preferentially cleaved into cellobiose (G2) and cellopentaose (G5) was cleaved into G2 + cellotriose (G3).