Isolation and genetic dissection of an eukaryotic replicon that supports autonomous DNA replication

Abstract

Maintenance of genome integrity requires that chromosomes be accurately and faithfully replicated. We are using Tetrahymena thermophila as a model system for studying the initiation and regulation of eukaryotic DNA replication. This organism contains a diploid micronucleus and polyploid macronucleus. During macronuclear development, the five diploid chromosomes of the micronucleus are fragmented into 280 macronuclear minichromosomes that are subsequently replicated to ~45 copies. In stark contrast, the 21 kb ribosomal DNA minichromosome (rDNA) is amplified from 2 to 10,000 copies in the same nucleus. Previous characterization of the rDNA replicon has led to the localization of its origin and the cis-acting regulatory determinants to the 1.9 kb 5'non-transcribed spacer region. The objective of this study was to identify and characterize non-rDNA origins of replication in Tetrahymena. This will help determine the underlying basis for differential regulation of rDNA and non-rDNA origins during development, as well as provide a better understanding of the organization of eukaryotic replicons. To this effect, I developed a DNA transformation assay that I used to isolate new Tetrahymena replication origins. A 6.7 kb non-rDNA fragment, designated TtARS1, was shown to support stable autonomous replication of circular plasmids in Tetrahymena. Genetic dissection revealed that TtARS1 contains two independent replicons, TtARS1-A and TtARS1-B. Full TtARS1-A function requires a minimal sequence of 700 bp, and two small regions in this fragment have been shown to be essential for origin function. TtARS1-B replicon function was localized to a 1.2 kb intergenic segment that contains little sequence similarity to TtARS1-A. Both non-rDNA replicons lack sequence similarity to the rDNA 5' NTS, suggesting that each replicon interact with a different set of regulatory proteins. This study indicates that the rDNA and the non-rDNA replicons have a modular organization, containing discrete, cis-acting replication determinants.