Pleiotropic effect of a point mutation in the yeast SUP4-o tRNA gene: in vivo pre-tRNA processing in S. cerevisiae

Pre-tRNA 3'-processing in Saccharomyces cerevisiae. Purification and characterization of exo- and endoribonucleases

We investigated ribonucleases from Saccharomyces cerevisiae which are active in pre-tRNA 3'-processing in vitro. Two pre-tRNA 3'-exonucleases with molecular masses of 33 and 60 kDa, two pre-tRNA 3'-endonucleases with molecular masses of 45 kDa/60 kDa and 55 kDa and 70-kDa 3'-pre-tRNase were purified from yeast whole cell extracts by several successive chromatographic purification steps. The purified exonucleases are non-processive 3'-exonucleases that catalyze the exonucleolytic processing of 3'-trailer sequences of pre-tRNAs to produce mature tRNAs. The 45-kDa/60-kDa 3'-endonuclease is tRNA-specific and catalyzes the processing of pre-tRNAs in a single endonucleolytic step. Two isoenzymes of this activity (p45 and p60) were identified by chromatography. The second endonuclease, p55, is dependent on monovalent ions and cleaves about three nucleotides downstream the mature 3'-end. All of the purified 3'-pre-tRNases accept homologous as well as heterologous pre-tRNA substrates. Pre-tRNAs carrying a 5'-leader are processed with almost the same efficiency as those lacking this 5'-leader. Mature tRNAs carrying the CCA 3'-sequence and tRNA pseudogene products carrying mutations in the mature domain are processed by the 3'-exonucleases, not by the 3'-endonucleases. The specific endonuclease p45/p60 discriminates between UUUOH as a 3'-flank, which is cleaved, and the CCA 3'-end of mature tRNAs, which is not cleaved. This study suggests that several 3'-pre-tRNases are active on tRNA precursors in vitro and might therefore in pre-tRNA 3'-processing in yeast, partly in a cooperative manner.

A very poorly expressed tRNA(Ser) is highly concentrated together with replication primer initiator tRNA(Met) in the yeast Ty1 virus-like particles

The analysis of the tRNAs associated to the virus-like particles produced by the Ty1 element revealed the specific packaging of three major tRNA species, in about equal amounts: the replication primer initiator tRNA(Met), the tRNA(Ser)AGA and a tRNA undetected until now as an expressed species in yeast. The latter tRNA is coded by the already described tDNA(Ser)GCT. This tRNA is enriched more than 150 fold in the particles as compared to its content in total cellular tRNA where it represents less than 0.1% (initiator tRNA(Met) and tRNA(Ser)AGA being 11 and 4 fold enriched respectively). This tRNA is the only species coded by the tDNA(Ser)GCT gene which is found in three copies per genome since no other corresponding expressed tRNA could be detected. This gene is thus very poorly expressed. The high concentration of tRNA(Ser)GCU in the particles compared to its very low cellular content led us to consider its possible implication in Ty specific processes.

Rapid transfer of small RNAs from a polyacrylamide gel onto a nylon membrane using a gel dryer

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