Metagenomic resources representing ruminal bacteria were screened for novel exocellulases using a robotic, high-throughput screening system, the novel CelEx-BR12 gene was identified and the predicted CelEx-BR12 protein was characterized. The CelEx-BR12 gene had an open reading frame (ORF) of 1140 base pairs that encoded a 380-amino-acid-protein with a predicted molecular mass of 41.8 kDa. The amino acid sequence was 83% identical to that of a family 5 glycosyl hydrolase from Prevotella ruminicola 23. Codon-optimized CelEx-BR12 was overexpressed in Escherichia coli and purified using Ni-NTA affinity chromatography. The Michaelis-Menten constant (K-m value) and maximal reaction velocity (V-max values) for exocellulase activity were 12.92 mu M and 1.55 x 10(-4) mu mol min(-1), respectively, and the enzyme was optimally active at pH 5.0 and 37 degrees C. Multifunctional activities were observed against fluorogenic and natural glycosides, such as 4-methylumbelliferyl-beta-D-cellobioside (0.3 U mg(-1)), CMC (105.9 U mg(-1)), birch wood xylan (132.3 U mg(-1)), oat spelt xylan (67.9 U mg(-1)), and 2-hydroxyethyl-cellulose (26.3 U mg(-1)). Based on these findings, we believe that CelEx-BR12 is an efficient multifunctional enzyme as endocellulase/exocellulase/xylanase activities that may prove useful for biotechnological applications. (C) 2013 Elsevier Inc. All rights reserved.