tRNA Genes Affect Multiple Aspects of Local Chromosome Behavior.

Abstract

In eukaryotic organisms the tRNA genes, which exist in multiple copies, are spread throughout the genome. Although it had been assumed that these genes are spread throughout the nucleus, it has recently shown that tRNA genes are actually clustered at the nucleolus. Since the tRNA genes seem to play a role in organizing the entire genome, their transcription may have dramatic influences. The three studies presented here were undergone to provide insight into these influences and their maintenance. The first study provides evidence that tRNA gene transcription influences recombination in the genome. The study examined two different circumstances. The first situation was used to tested if the transcription of repetitive tRNA genes influenced their tendency to undergo homologous recombination and the second situation was used to test how a tRNA gene influences the homologous recombination of two nearby repetitive sequences. It was determined that two transcriptionally active tRNA genes had a five times greater rate of recombination than if one or both tRNA genes were inactivated, however tRNA gene transcription had no discernible influence over the recombination between nearby repetitive elements. This could have implications in the human genome, which contains over 1 million RNA polymerase III transcribed SINE elements. The second study explores how tRNA gene transcription influences their clustering and localization at the nucleolus. This study demonstrated that the condensin complex associates with tRNA genes and physically interacts with RNA polymerase III transcription factors. Condensin’s interactions with these two elements may lead to the clustering and localization of the tRNA genes, which could play an important role in the organization of the entire genome in eukaryotes. The third study found a gene deletion, mod5Δ, that results in the alleviation of silencing near tRNA genes. The results of this study indicate that Mod5p’s catalytic activity is necessary for tgm silencing. The protein was also shown to be present at tRNA genes and seems to maintain the silencing in cis. These results indicate that Mod5p performs an activity necessary for the repression of pol II transcription and this may have implications for the regulation of many genes in higher eukaryotes.Ph.D.Cellular & Molecular BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60716/1/mhyatt_1.pd