Jean M. Ohrt

Conformation and possible role of hypermodified nucleosides adjacent to 3′-end of anticodon in tRNA: N-(purin-6-ylcarbamoyl)-L-threonine riboside

The crystal structure of N-(purin-6-ylcarbamoyl)-L-threonine riboside was determined from three-dimensional x-ray diffraction data. The N6-substituent is distal (trans) to the imidazole ring, leading to a bifurcated hydrogen interaction involving two intramolecular contacts with the hydrogen on N(threonine): a hydrogen bond to N(1) of adenine and a close contact to the hydroxyl oxygen of threonine. The conformation of the molecule and the internal hydrogen bond completely block the two sites N6-H and N1 of adenine from taking part in the Watson-Crick base pairing. This inability to base pair according to the Watson-Crick scheme appears as a common structural feature in all modified bases adjacent to the 3′-end of anticodons. These results, along with Cricks hypothesis for codon recognition, suggest that the hypermodified bases adjacent to the anticodons may be important in (i) preventing the misreading of the codons by bases adjacent to anticodons and (ii) promoting a single stranded conformation for the anticodon loops.

Conformation of N-(purin-6ylcarbamoyl) glycine, a hypermodified base in tRNA

The crystal structure of the potassium salt of N-(purin-6ylcarbamoyl) glycine was determined from three-dimensional X-ray diffraction data. The N6-substituent is distal (trans) to the imidazole ring, forming an intramolecular hydrogen bond N(glycine) -H---N(1)adenine. This conformation of the N6-substituent is typical of ureidopurines, and blocks the two sites N6-H and N1 of adenine that are normally utilized for complementary base-pairing in the double helical regions of nucleic acids; the internal hydrogen bonding further enhances the shielding of N1. This blocking of N6-H and N1 may be important in enhancing the single stranded conformation of the anticodon loop of tRNA and in preventing the modified adenosine adjacent to the anticodon from taking part directly in codon-anticodon interaction through the complementary base pairing.