beta-Glucuronidase activity (encoded by the gus gene) has been characterized for the first time from Ruminococcus gnavus E1, an anaerobic bacterium belonging to the dominant human gut microbiota. beta-Glucuronidase activity plays a major role in the generation of toxic and carcinogenic metabolites in the large intestine, as well as in the absorption and enterohepatic circulation of many aglycone residues with protective effects, such as lignans, flavonoids, ceramide and glycyrrhetinic acid, that are liberated by the hydrolysis of the corresponding glucuronides. The complete nucleotide sequence of a 4537 bp DNA fragment containing the beta-glucuronidase locus from R. gnavus E1 was determined. Five ORFs were detected on this fragment: three complete ORFs (ORF2, gus and ORF3) and two partial ORFs (ORF4 and ORF5). The products of ORF2 and ORF3 show strong similarities with many beta-glucoside permeases of the phosphoenolpyruvate : beta-glucoside phosphotransferase systems (PTSs), such as Escherichia coli BglC, Bacillus subtilis BglP and Bacillus halodurans PTS Enzyme II. The product of ORF5 presents strong similarities with the amino-terminal domain of Clostridium acetobutylicum beta-glucosidase (bglA). The gus gene product presents similarities with several known beta-glucuronidase enzymes, including those of Lactobacillus gasseri (69%), E. coli (61%), Clostridium perfringens (59%) and Staphylococcus aureus (58%). By complementing an E. coli strain in which the uidA gene encoding the enzyme was deleted, it was confirmed that the R. gnavus gus gene encodes the beta-glucuronidase enzyme. Moreover, it was found that the gus gene was transcribed as part of an operon that includes ORF2, ORF3 and ORF5.