3 Processing of Yeast Cytoplasmic and Mitochondrial Precusor tRNAs

I. INTRODUCTION The purpose of this chapter is to describe the processing of pre-tRNAs in yeast, integrating information on processing in the nucleus and mitochondria. Precursor tRNAs produced from nuclear genes and mitochondrial genes have the same general requirements: Various activities are needed to remove 5´leaders and 3´trailers, to add the CCA end, and to catalyze numerous base modifications (Fig. 1). In addition, a subset of nuclear pre-tRNAs have introns that must be removed, although no such activities are required for the biogenesis of any yeast mitochondria1 tRNA. Some yeast nuclear tRNA genes are transcribed together in dimeric pairs (Schmidt et al. 1980), and mitochondria1 tRNAs are transcribed with other tRNAs (Palleschi et al. 1984b; Martin et al. 1985b; Bardonne et al. 1987; Francisci et al. 1987), with ribosomal RNAs (Osinga et al. 1984; Palleschi et al. 1984a), with mRNAs (Miller et al. 1983; Zassenhaus et al. 1984), or with the RNasc P RNA (Shu and Martin 1991). Although these polycistronic transcripts are processed by a variety of activities, only those directly involved in tRNA recognition and processing are considered here. We have not attempted to review tRNA gene organization or transcription, as these topics have been covered elsewhere. The reader is referred to Guthrie and Abelson (1982) for a review of yeast nuclear tRNA genes, to Thuriaux and Sentenac (this volume) for a review of nuclear tRNA gene transcription, and to Tzagoloff and Myers (1986) for a review of mitochondria1 tRNA genes. In general, we have...