STRUCTURE OF THE ARCHAEAL TRANSFER-RNA NUCLEOSIDE-G-ASTERISK-15 OSYL-1H-PYRROLO[2,3-D]PYRIMIDINE-5-CARBOXIMIDAMIDE (ARCHAEOSINE))

A number of post-transcriptional modifications in tRNA are phylogeneti cally characteristic of the bacterial, eukaryal, or archaeal domains, both with respect to sequence location and molecular structure at the nucleoside level. One of the most distinct such modifications is nucle oside G, located in archaeal tRNA at position 15, which in bacterial and eukaryal tRNAs is a conserved site involved in maintenance of the dihydrouridine loop-T-loop tertiary interactions. G occurs widely in nearly every branch of the archaeal phylogenetic domain, in contrast t o its absence in all reported bacterial and eukaryal tRNA sequences. T he structure of G- 15 is osyl-1H-pyrrolo[2,3-d]pyrimidine-5-carboximi damide (7-formamidino-7-deazaguanosine), which is a non-purine, non-py rimidine ribonucleoside; its structure thus reflects extensive modific ation beyond the guanine-15 specified by corresponding gene sequences. The structure was established by mass spectrometry, and in particular from collision-induced dissociation mass spectra of derivatives forme d by microscale permethylation, and is confirmed by chemical synthesis .