Dihydrouridine

Dihydrouridine (abbreviated as D, DHU, or UH2) is a pyrimidine nucleoside which is the result of adding two hydrogen atoms to a uridine, making it a fully saturated pyrimidine ring with no remaining double bonds. D is found in tRNA and rRNA molecules as a nucleoside; the corresponding nucleobase is 5,6-dihydrouracil.
Because it is non-planar, D disturbs the stacking interactions in helices and destabilizes the RNA structure. D also stabilizes the C2’-"endo" sugar conformation, which is more flexible than the C3’-"endo" conformation; this effect is propagated to the 5’-neighboring residue. Thus, while pseudouridine and 2’-O-methylations stabilize the local RNA structure, D does the opposite.
The tRNAs of organisms that grow at low temperatures (psychrophiles) have high 5,6-dihydrouridine levels (40-70% more on average) which provides the necessary local flexibility of the tRNA at or below the freezing point.