The structure of the unknown modified nucleoside Q, which is present in the first position of the anticodons of Escherichia coli tRNA Tyr, tRNA His, tRNA Asn, tRNA Asp, is proposed to be 7-(4,5-cis-dihydroxy-1-cyclopenten-3-ylaminomethyl)-7-deazaguanosine (1). The structure of Q was deduced by means of its uv absorption, mass spectrometry, proton magnetic resonance spectroscopy, and studies of its chemical reactivity. The structure of Q is unique since it is a derivative of 7-deazaguanosine having cyclopentenediol in the side chain at the C-7 position. This is the first example of purine skeleton modification in a nucleoside from tRNA.

Queuosine (Q), 7-(((4,5-cis-dihydroxy-2-cyclopentene-1-yl)-amino)methyl)-7-deazaguanosine, and Q derivatives usually replace guanosine in the anticodon of tRNAsGUN of eubacteria and of cytoplasmic and mitochondrial tRNAs of lower and higher eucaryotes except yeasts. Q appears to be synthesized de novo exclusively in eubacteria, and the free-base queuine serves as a nutrient factor for eucaryotes. Recently, a Q derivative, oQ, containing a 2,3-epoxy-4,5-dihydroxycyclopentane ring, has been identified in Escherichia coli tRNATYF. Here we show that oQ is formed when E. coli or SalmoneUa typhimurium is grown in glucose-salt medium. The formation of oQ was independent of molecular oxygen, and oQ-tRNAs were converted to Q-tRNAs by adding cobalamin to the growth medium. Under strictly anaerobic conditions, considerable amounts of Q were present in E. coli and S. typhimurium tRNAs when the bacteria were grown in the presence of cobalt ions with glycerol as the carbon source and fumarate as the electron acceptor. Under these conditions, the biosynthesis of cobalamin was induced. The results suggest that oQ is derived from ribose and that oQ is finally reduced to Q by a cobamide-dependent enzyme.

Queuine, a modified base found in transfer RNA, appears to be a new dietary factor because (i) previous studies have shown that mice require it for the expression of queuine-containing transfer RNA's but apparently do not synthesize it, and (ii) significant amounts of free queuine are present in common plant and animal food products.

A photoadduct is formed between the adenine (A) and thymine (T) bases of the deoxydinucleoside monophosphate d(TpA) when it is irradiated at 254 nanometers in aqueous solution. Treatment of the photoadduct with acid converts it specifically into a fluorescent hydrolysis product, C7H7N3O, incorporating the position-8 carbon of adenine and the methyl group of thymine. Isolation of the fluorescent hydrolysis product from acid hydrolyzates of oligo- and polydeoxyribonucleotides has shown that the photoadduct is formed by ultraviolet irradiation of d(pTpA), d(TpApT), d(TpApTpA), poly(dA-dT), and both single- and double-stranded DNA.

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