RNA Catalyzed Lariat Formation from Yeast Mitochondrial Pre-Ribosomal RNA

Mitochondria of Saccharomyces cerevisiae contain a circular DNA of around 80 kb. It comprises a limited set of genes which code for 2 ribosomal RNAs (rRNA), approximately 25 tRNAs, an RNA involved in tRNA processing, 6 proteins which are part of respiratory chain complexes, a protein (varl) associated with the mitochondrial ribosome and proteins (maturases), which are involved in RNA splicing (for review see Dujon, 1981). In the strain S. cerevisiae KL14-4A three of these genes are interrupted by intervening sequences (introns): the genes coding for large rRNA, apocytochrome b and subunit I of cytochrome c oxidase (see Fig.1). Removal of introns from precursor RNAs requires the presence of a RNA splicing machinery. The mitochondrial splicing process differs in a number of aspects from splicing of precursor RNAs synthesized by RNA polymerase II: i) mitochondrial introns possess conserved nucleotide blocks, which by inducing specific folding of intron RNA are thought to facilitate the splicing process (Michel and Dujon, 1983; Waring and Davies, 1984). Mutations in such sequence elements have a cis-acting phenotype (Lamouroux et al. 1980). In contrast nuclear introns in higher eukaryotes do not contain sequence elements of this type that contribute to RNA splicing (Wieringa et al. 1984). ii) some mitochondrial introns contain long open reading frames in phase with the upstream exon. Translation of unspliced RNA by mitochondrial ribosomes leads to synthesis of hybrid proteins (maturases), which are involved in and contribute to intron removal (Lazowska et al. 1980). This is supported by the observation that nucleotide alterations leading to a deficient reading frame can be complemented in trans by the wildtype allele (Lamouroux et al. 1980).