The nutrient factor queuine: biosynthesis, occurrence in transfer RNA and function.

Queuine, 7-(( (4,5-cis-dihydroxy-2-cyclopenten-1-yl)-amino]-methyl)-7-deazagu ani ne is synthesized de novo only in eubacteria and is preseent in place of guanine 34 in specific tRNAs containing anticodones GUN where N is one of the four canonical nucleotides. The biosynthetic pathway starting with GTP shares common steps with that of pteridines and riboflavin, and involves iron ions and a 'vitamin B12' coenzyme. Lower and higher eukaryotes are supplied with queuine by nutrition or the intestinal flora. The modification of tRNA with queuine is tissue specific and depends on the metabolic state of cells and tissues. Starvation for queuine and/or Q-deficiency in tRNA causes a few specific changes in the pattern of protein synthesis involving lactate dehydrogenases and cytochromes.