Saccharomyces cerevisiae SUP53 tRNA gene transcripts are processed by mammalian cell extracts in vitro but are not processed in vivo.

We describe the results of our studies of expression of a Saccharomyces cerevisiae amber suppressor tRNA(Leu) gene (SUP53) in mammalian cells in vivo and in cell extracts in vitro. Parallel studies were carried out with the wild-type (Su-) tRNA(Leu) gene. Extracts from HeLa or CV1 cells transcribed both tRNA(Leu) genes. The transcripts were processed correctly at the 5' and 3' ends and accurately spliced to produce mature tRNA(Leu). Surprisingly, when the same tRNA(Leu) genes were introduced into CV1 cells, only pre-tRNAs(Leu) were produced. The pre-tRNAs(Leu) made in vivo were of the same size and contained the 5'-leader and 3'-trailer sequences as did pre-tRNAs(Leu) made in vitro. Furthermore, the pre-tRNAs(Leu) made in vivo were processed to mature tRNA(Leu) when incubated with HeLa cell extracts. A tRNA(Leu) gene from which the intervening sequence had been removed yielded RNAs that also were not processed at either their 5' or 3' termini. Thus, processing of pre-tRNA(Leu) in CV1 cells is blocked at the level of 5'- and 3'-end maturation. One possible explanation of the discrepancy in the results obtained in vivo and in vitro is that tRNA biosynthesis in mammalian cells involves transport of pre-tRNA from the site of its synthesis to a site or sites where processing takes place, and perhaps the yeast pre-tRNAs(Leu) synthesized in CV1 cells are not transported to the appropriate site.