Control of Gene Expression in the Biosynthesis of Mitochondrial Ribosomes

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Despite the universality of ribosomes in procaryotic and eucaryotic cells, the majority of our knowledge of their structure, function and synthesis has been developed through decades of intensive study of bacterial ribosomes, predominantly in E. coli. In contrast to bacteria, eucaryotic cell maintain distinctly different sets of ribosomes for protein synthesis in cytoplasm, chloroplasts, and mitochondria. An important research objective is to understand the features of eucaryotic ribosomes that represent special adaptations to complex genomic organization and subcellular comparmentalization for eucaryotic cells. The yeast Saccharomyces cerevisiae is a fertile experimental system for the analysis of both organellar and cytoplasmic ribosomes within a single cell. Several laboratories are studying yeast cytoplasmic ribosomes, our goal is to contribute complementary information about yeast mitochondrial ribosomes and their constituents. The principal function of the mitochondrial translation system in yeast is to produce a small number of subunits for the energy-transducing complexes of the inner membrane. To accomplish this task, both the nuclear mitochondrial genes are used to encode the requisite RNAs and proteins of the translational apparatus. With the exception of one protein of the small ribosomal subunit (VAR1), all of the ribosomal proteins are encoded by 60-70 different nuclear genes. Of the four such genes characterized by us MRP7 and MRP20 are particularly intriguing because they encode proteins that are related to the EL27 and EL23 proteins, respectively, of the E. coli 50S ribosomal subunit. Both MRP& and MRP20 are essential proteins in yeast, and there is strong experimental evidence for close physical proximity between EL27 and the peptidyltransferase center of the E. coli ribosome. EL23 is a primary rRNA binding protein, which may also be located near the peptidvltransferase center. The research objectives of this proposal are based on the contention that MRP7 and MRP20, like their bacterial counterparts, are likely to be close to an important catalytic center of the ribosome. We intend to use a combined molecular genetic and biochemical approach to : 1) gain additional information about the synthesis, structure, and function of the MRP7 and MRP20 or both. The research will provide new insight into the structure and function of proteins that are located in a highly conserved functional domain of the ribosome.