Differential gene expression analysis was performed in monoxenic mice colonized with Ruminococcus gnavus strain E1, a major endogenous member of the gut microbiota. RNA arbitrarily primed-PCR fingerprinting assays allowed to specifically detect the invivo expression of the aga1 gene, which was further confirmed by RT-PCR. The aga1 gene encoded a protein of 744 residues with calculated molecular mass of 85,207Da. Aga1 exhibited significant similarity with previously characterized alpha-Galactosidases of the GH 36 family. Purified recombinant protein demonstrated high catalytic activity (1047Umg(-1)) and efficient p-nitrophenyl-alpha-d-galactopyranoside hydrolysis [k(cat)/K(m)=35.1158.82s(-1)mM(-1) at 55degreesC and k(cat)/K(m)=17.484.25s(-1)mM(-1) at 37degreesC].