Insight into Translational Activation in Yeast Mitochondria

Mitochondrial function depends on over a thousand proteins, of which the majority are nuclear DNA-encoded and approximately one percent are mitochondrial DNA-encoded. The mitochondrial DNA of Saccharomyces cerevisiae contains eight protein-encoding genes, seven of which are required for proper function of the respiratory complexes and one encodes a ribosomal protein. The bigenomic nature of the oxidative phosphorylation complexes requires coordinated expression and regulation from both the nuclear and the mitochondrial genomes. It is currently unclear how this regulatory network operates. However, it is thought that nuclear genome-encoded messengers localized to the mitochondria aid in this coordination. A family of proteins termed mitochondrial translational activators has been shown to control the expression of all mitochondrial-encoded protein genes in a gene-specific manner. Evidently, each mitochondrial mRNA is regulated by a specific protein or a subset of proteins that permits translation of that transcript. If these factors are absent, there will be no synthesis of the polypeptide, leading to a respiratory deficient cell. Although a functional link between translational activators and the mitochondrial genes they control has long been established genetically, the activation mechanism is entirely unknown. This study focuses on the mechanism of activation of two representative members of this translational activator family, Pet111p and Cbp1p. These activators are required for the expression of COX2 and COB, respectively. Translational activators have been shown to have multiple functions including, transcript stabilization and mRNA localization to the membrane, as well as to the translation machinery. Current genetic data suggests that both Pet111p and Cbp1p interact with RNA targets in the 5′-untranslated regions of COX2 and COB, respectively. However, neither the identity of these sites, nor has the ability of these proteins to interact with RNA ever been demonstrated. The objective of this study is to characterize the functional mechanism of translational activation for both Pet111p and Cbp1p and ultimately learn how they aid in the coordination of dual-genomic expression