CD spectra of 5-methyl-2-thiouridine in tRNAfMet from an extreme thermophile

5-Methyl-2-thiouridine (S) in $tRNAfMet$ from an extreme thermophile is located in the TψC region, replacing T, and has a positive CD band centered at 310 nm. Upon heating, the profiles of the change in this band were similar to the UV melting profiles of the change monitored at 260 nm. This strongly suggests a close relation between heat denaturation of the tRNA and the conformation of the S base. Oligonucleotides containing S showed negative CD bands at 320–330 nm, like the monomer S itself, but when the 3′–2/5 fragment containing S formed a complex with the complementary 5′–3/5 fragment, a positive CD band appeared at 310 nm. These results suggest that combination of the TψC loop containing S with the hU loop is necessary for the positive band of S at 310 nm. S may serve to strengthen the association of the TψC loop with the hU loop in tRNA of the thermophile.