In sequenced genomes, genes belonging to the cluster of orthologous group COG1041 are exclusively, and almost ubiquitously, found in Eukaryota and Archaea but never in Bacteria. The corresponding gene products exhibit a characteristic Rossmann fold, S-adenosylmethionine-dependent methyltransferase domain in the C terminus and a predicted RNA-binding THUMP (thiouridine synthases, RNA methyltransferases, and pseudouridine synthases) domain in the N terminus. Recombinant PAB1283 protein from the archaeon Pyrococcus abyssi GE5, a member of COG1041, was purified and shown to behave as a monomeric 39-kDa entity. This protein (EC 184.108.40.206), now renamed (Pab)Trm-G10, which is extremely thermostable, forms a 1:1 complex with tRNA and catalyzes the adenosylmethionine-dependent methylation of the exocyclic amino group (N(2)) of guanosine located at position 10. Depending on the experimental conditions used, as well as the tRNA substrate tested, the enzymatic reaction leads to the formation of either N(2)-monomethyl (m(2)G) or N(2)-dimethylguanosine (m(2)(2)G). Interestingly, (Pab)Trm-G10 exhibits different domain organization and different catalytic site architecture from another, earlier characterized, tRNA-dimethyltransferase from Pyrococcus furiosus ((Pfu)Trm-G26, also known as (Pfu)Trm1, a member of COG1867) that catalyzes an identical two-step dimethylation of guanosine but at position 26 in tRNAs and is also conserved among all sequenced Eukaryota and Archaea. The co-occurrence of these two guanosine dimethyltransferases in both Archaea and Eukaryota but not in Bacteria is a hallmark of distinct tRNAs maturation strategies between these domains of life.