Abstract 9406322 Tetrahymena thermophila is the only organism in which the rDNA is not redundant in the germline genome and in which complete replacement of the functional rDNA has been achieved by this laboratory. It thus represents a unique model system in which to study rRNA function by genetic means. We will continue and expand our study of rRNA function in several ways. We will continue our analysis of the D8 variable region, a conserved region which preliminary experiments indicate is essential for cell growth and will then analyze three other regions to determine if the sequence and structural features that are important for cell growth in the D8 region are found in those regions. We will do phylogenetic analysis of comparative sequences of the D8 region in Tetrahymena chosen to represent groups with varying evolutionary distances from T. thermophila. We will test whether the conserved features of the D8 region reside in their secondary or tertiary structures by selectively altering limited sequences and determining their ability to support cell growth after transformation. We will continue several approaches in order to further define the possible function of the D8 region and the peptidyl transferase center. We will also study the effects of rDNA mutations on nucleolar organization. The results of this study should give a greatly expanded view of the function of rRNA in eukaryotic cells. %%% The ribosomal RNAs are among the most characterized molecules in the cell. The complete nucleotide sequences of these molecules are known from nearly 200 species of organisms including both eukaryotes and prokaryotes as well as a number of chloroplasts and nitochondria. Despite this knowledge, very little is known about the function of these ubiquitous and important molecules. Essentially nothing is known about how rRNAs are involved in protein synthesis or other cellular activities. They may actually be involved in the catalytic process of the ribososme, thus repre senting the largest and most ancient ribozymes. Past studies of rRNA function have been limited in part by diffculties in genetic manipulations. The rRNA genes are present in multiple copies in almost all experimental organisms. Tetrahymena thermophila is the only organism in which the rDNA is not redundant in the germline genome and in which complete replacement of the functional rDNA has been achieved. It thus represents a unique model system in which to study rRNA function by genetic means. ***
