The crystal structure of HIV reverse-transcription primer tRNA(Lys,3) shows a canonical anticodon loop

We have solved to 3.3 Angstrom resolution the crystal structure of the HIV reverse-transcription primer tRNA(Lys,3). The overall structure is exactly comparable to the well-known L-shape structure first revealed by yeast tRNA (Phe), In particular, it unambiguously shows a canonical anticodon loop, Th is contradicts previous results in short RNA fragment studies and leads us to conclude that neither frameshifting specificities of tRNA(Lys) nor tRNA( Lys,3) primer selection by HIV are due to a specific three-dimensional anti codon structure. Comparison of our structure with the results of an NMR stu dy on a hairpin representing a nonmodified anticodon stem-loop makes plausi ble the conclusion that chemical modifications of the wobble base U34 to 5- methoxycarbonyl-methyl-2-thiouridine and of A37 to 2-methylthio-N-6-threony lcarbamoyl-adenosine would be responsible for a canonical 7-nt anticodon-lo op structure, whereas the unmodified form would result in a noncanonical UU U short triloop. The hexagonal crystal packing is remarkable and shows tigh t dimers of tRNAs forming a right-handed double superhelix. Within the dime rs, the tRNAs are associated head-to-tail such that the CCA end of one tRNA interacts with the anticodon of the symmetry-related tRNA. This provides u s with a partial view of a codon-anticodon interaction and gives insights i nto the positioning of residue 37, and of its posttranscriptional modificat ions, relative to the first base of the codon.