Dynamic structures and functions of transfer ribonucleic acids from extreme thermophiles.

tRNA species from an extreme thermophile T. thermophilus that grows up to 85 degrees C have been found to be more thermostable than those from moderate thermophiles and mesophiles. Such thermostability of T. thermophilus tRNA species is partly due to the high contents of G.C base pairs in the stem regions. In addition, a novel modified nucleoside s2T has been found that substitutes T in position 54. The extent of 2-thiolation of T(54) has been found to depend on environmental temperatures from 50 to 80 degrees C. Two tRNA(Ile) species have been isolated from T. thermophilus HB8, tRNA(1aIle) with s2T(54) and tRNA(1bIle) with T(54), which have the identical nucleotide sequence except for position 54. However, the melting temperature of tRNA(1aIle) is higher by 3 degrees C than that of tRNA(1bIle). This clearly indicates that the 2-thiolation of T(54) contributes directly to the thermostability of T. thermophilus tRNA species. Proton NMR analyses have shown that the nucleoside s2T is "rigid" and predominantly takes the C3'-endo-gg-anti form of A-RNA, because of the steric effect of the bulky 2-thiocarbonyl groups and the 2'-hydroxyl group. Thus, the inherent rigidity of s2T in position 54 significantly enhances the stability of the tertiary structure of tRNA. In protein synthesis of T. thermophilus, s2T(54)-bearing tRNA and T(54)-bearing tRNA species are selectively utilized depending on environmental temperature. In the anticodons of major tRNA species from T. thermophilus, G or C exclusively appears in the first position, and GGN and CCN are favored over synonymous GCN or CGN. These characteristic anticodon sequences correspond to the characteristic codon usage in thermophile genes.