The expression of prokaryotic tRNA genes in frog oocytes

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Abstract
A tRNA gene cluster in Salmonella tvphimurium includes the genes for tRNAArg, tRNAHis, tRNALeu and tRNAPro. DNA clones were constructed with different portions of this tRNA gene cluster. These clones were microinjected into the nuclei of Xenopus laevis oocytes and assayed for expression. Two of the bacterial tRNA genes (tRNAArg and tRNAPro) are transcribed at high rates and the primary transcripts are processed into mature tRNAs. Transcription and processing are largely independent of whether the two genes are injected individually or as part of a tRNA gene cluster. A third tRNA gene (tRNALeu) is expressed less efficiently. Synthesis of this tRNA is totally abolished by a deletion removing 22 bp in the first half of the tRNALeu coding sequence. The expression of the fourth tRNA gene (tRNAHis) is very inefficient and dependent upon the gene organisation within the injected DNA. No significant tRNA synthesis is detected upon injection of a clone containing only the tRNAHisgene. Evidence is presented suggesting that the impaired expression of the tRNAHis gene is not caused by inefficient transcription, but rather by defective processing of the primary transcript. The prokaryotic tRNAs synthesized in the oocytes show a modification pattern that is specific of eukaryotic tRNAs. Overall, our results are consistent with the hypothesis that the intragenic signals for eukaryotic tRNA gene transcription have appeared early in evolution for reasons other than gene expression.