Translation in Yeast Mitochondria

Abstract

Mitochondria make a small number of their own proteins. They do this employing a translational apparatus that is clearly unusual (see Benne and Sloof, 1987 for review). A different genetic code; a low number of tRNAs, many of them unconventional; diminutive rRNAs; a lack of a typical 5S rRNA; mRNAs with shrunken, or no leader sequences (mammals); mRNAs with long leader and trailer sequences (yeast) and a large number of what appear to be rnRNA-specific translation factors (yeast) are just some of the hallmarks that distinguish this machinery from typical pro- or eukaryotic cell sap systems. These features have both livened up the study of mitochondrial translation in the past few years and have contributed to our understanding of such diverse biological questions as decoding mechanisms, the definition of the minimal requirements for rRNA and tRNA function and the evolution of ribosomes. This knowledge has been acquired despite the undoubted disadvantage that no one has yet managed to develop an mRNA-dependent translation system from mitochondria. There are two reasons for this fortunate situation: 1 Much has been learnt from comparisons of mitochondrially-coded translational components (mostly rRNAs and tRNAs) in various organisms. These reveal a remarkable and informative diversity that is perhaps best explained in terms of the low demands the cell puts on these components. Generally speaking, mitochondrial translational systems produce a limited number of proteins at relatively constant rate and consequently may be able to evolve more rapidly than systems subject to complex translational controls and producing a large number of proteins with maximal efficiency (Borst, 1981). 2 Mitochondrial translation is very amenable to genetic analysis. This is especially so in a facultative anaerobe, such as Saccharomyces cerevisiae, which when growing on fermentable carbon sources, can dispense with respiratory function, thus allowing the ready selection of mutations affecting various steps in mitochondrial gene expression. Analysis is further facilitated by the fact that, in contrast to other genetic systems, genes for rRNAs and tRNAs each occur in only one copy per mitochondrial genome. Thus, mutations conferring antibiotic resistance, or heat/cold sensitivity to mitochondrial protein synthesis have turned out to lie within the rRNA genes and identify rRNA domains involved in the peptidyl transferase centre and in translational fidelity (Sor and Fukuhara, 1982, 1984; Kruszewska and Slonimski, 1984; Weiss-Brummer et al., 1987; Daignan-Fornier et al., 1988; Sakai et al., 1991). Second-site nuclear or mitochondrial suppressors of some of these mutations have also been isolated (Julou et al., 1984; Daignan-Fornier and Bolotin-Fukuhara, 1988; Valens et al., 1991) and these are likely to yield information on RNA-protein interactions within the ribosome and possibly also on long-range folding interactions within the rRNAs themselves.