Characterization of tRNA precursor splicing in mammalian extracts

Transcription of a Xenopus laevis tRNATyr gene and splicing of the transcript have been studied in HeLa cell extracts. This tRNATyr gene has a 13-base intervening sequence and is expressed as mature tRNA when transfected into mammalian cells. The tRNATyr gene is transcribed under conditions of low concentrations of magnesium and ATP, but is processed by splicing only when both of these cofactors are added at higher concentrations. The endonucleolytic activity of the tRNA-splicing system in the HeLa extract produces exons with 3'-phosphate and 5'-hydroxyl groups. The 3'-phosphate is retained during the ligation reaction and forms the phosphodiester bond in the mature tRNA. Retention of the 3'-phosphate during tRNA splicing differs from the more extensively studied process in yeast extracts where a phosphate group from an ATP cofactor is used to form the phosphodiester bond joining the exons. Thus, eucaryotic organisms can splice tRNA precursors by at least two distinguishable mechanisms.