HYDROLYSIS OF DIHYDROURIDINE AND RELATED-COMPOUNDS

Dihydrouridine is absent from the tRNA of almost all hyperthermophiles and most Archaea but is ubiquitous in the tRNA of Eubacteria and Euka ryotes. In order to investigate whether this could be due to instabili ty, the rate of ring opening of dihydrouridine was measured between 25 and 120 degrees C. The dihydrouridine ring is stable at 25 degrees C, but the half-life at 100 degrees C and pH 7 is 9.1 h, which is compar able to the doubling time of hyperthermophiles. This suggests an expla nation for the absence of dihydrouridine from the tRNA of hyperthermop hiles. The rates of ring opening of dihydrouracil, dihydrothymine, and 1-N-methyldihydrouracil were measured at 100 degrees C and pH 6-9, as were the equilibrium constants for ring closure of the ureido acids t o the dihydrouracils. The pH rate profiles for ring opening and ring c losing were calculated from the data. Possible roles for dihydrouracil s in the pre-RNA world are discussed.