An additional modified nucleoside is also found as a constituent of tRNA. This modified nucleoside is called pseudouridine (Figure 13.8) and is unusual in that this nucleoside contains a C-glycosidic bond rather than N-glycosidic bond as shown in the structure for pseudouridine. As with the modified bases, pseudouridine is formed only after UMP has been incorporated into the RNA. The level of pseudouridine in the urine is also an excellent measure of tRNA turnover. Ribothymidine is found as a minor component of tRNA. This derivative is formed by the methylation of uracil at C-5 by
S-adenosylmethionine as a component of tRNA rather than by the direct incorporation of a ribothymidine nucleotide into this RNA species.