The gene encoding RUM630-BX, a beta-xylosidase/arabinofuranosidase, was identified from activity-based screening of a cow rumen metagenomic library. The recombinant enzyme is activated as much as 14-fold (k(cat)) by divalent metals Mg2+, Mn2+ and Co2+ but not by Ca2+, Ni2+, and Zn2+. Activation of RUM630-BX by Mg2+ (t(0.5) 144s) is slowed two-fold by prior incubation with substrate, consistent with the X-ray structure of closely related xylosidase RS223-BX that shows the divalent-metal activator is at the back of the active-site pocket so that bound substrate could block its entrance. The enzyme is considerably more active on natural substrates than artificial substrates, with activity (k(cat)/K-m) of 299 s(-1) mM(-1) on xylotetraose being the highest reported. Published by Elsevier Inc.