A metagenomic library consisting of 3,024 bacterial artificial chromosome clones was prepared in Escherichia coli DH10B with high-molecular-weight DNA extracted from red soil in South China. A novel cellulase gene with an open reading frame of 1,332 bp, cel5G, encoding an endo-beta-1,4-glucanase was cloned using an activity-based screen. The deduced enzyme, Cel5G, belongs to the glycosyl hydrolase family 5 and shares < 39% identity with endoglucanases in the GenBank database. cel5G was expressed in E. coli BL21, and the recombinant enzyme Cel5G was purified to homogeneity for enzymatic analysis. Cel5G hydrolyzed a wide range of beta-1,4-, beta-1,3/beta-1,4-, or beta-1,3/beta-1,6-linked polysaccharides, amorphous cellulose, filter paper, and microcrystalline cellulose. Its highest activity was in 50 mM citrate buffer, pH 4.8, at 50A degrees C. Cel5G is stable over a wide range of pH values (from 2.0 to 10.6) and is thermally stable under 60A degrees C. It is highly tolerant and active in high salt concentrations and is stable in the presence of pepsin and pancreatin. The K (m) and V (max) values of Cel5G for carboxymethyl cellulose are 19.92 mg/ml and 1,941 mu mol min(-1) mg(-1), respectively. These characteristics indicate that Cel5G has potential for industrial use.