The modified nucleoside 1-methyladenosine (m1A) is found in the T-loop of many tRNAs from organisms belonging to the three domains of life (Eukaryota, Bacteria, Archaea). In the T-loop of eukaryotic and bacterial tRNAs, m1A is present at position 58, whereas in archaeal tRNAs it is present at position(s) 58 and/or 57, m1A57 being the obligatory intermediate in the biosynthesis of 1-methylinosine (m1I57). In yeast, the formation of m1A58 is catalysed by the essential tRNA (m1A58) methyltransferase (MTase), a tetrameric enzyme that is composed of two types of subunits (Gcd14p and Gcd10p), whereas in the bacterium Thermus thermophilus the enzyme is a homotetramer of the Trml polypeptide. Here, we report that the Trml enzyme from the archaeon Pyrococcus abyssi is also a homotetramer. However, unlike the bacterial site-specific Trml MTase, the P.abyssi enzyme is region-specific and catalyses the formation of m1A at two adjacent positions (57 and 58) in the T-loop of certain tRNAs. The stabilisation of P.abyssi Trml at extreme temperatures involves intersubunit disulphide bridges that reinforce the tetrameric oligomerisation, as revealed by biochemical and crystallographic evidences. The origin and evolution of m1A MTases is discussed in the context of different hypotheses of the tree of life.