Pasion Lab Research

The regulation of the replication of the genome is critical in the basic process of cell division. The genetic material must be duplicated correctly, completely, and at the appropriate time during the cell division cycle. Regulation of this process is essential to maintain genomic integrity and to prevent aneuploidy or genetic instability, which are hallmarks of cancer. Of further importance is understanding how the cell responds to replication stress induced by the encounter of the replication complex with a modified nucleotide or other form of aberrant DNA structure (DNA damage) or with a transcription complex.

We use the single-celled organism Schizosaccharomyces pombe, or fission yeast, as a model for the study of eukaryotic DNA replication. Fission yeast is an excellent model system to address questions relevant to S phase regulation because of the similarity of its cell cycle regulation to that of other eukaryotes, the tractability of its genetics and cell biology, the availability of molecular biology tools for manipulation, and the sequencing of its genome.

The focus of my laboratory centers on the analysis of cdc24+, an essential fission yeast gene that is required for genomic integrity and likely has a role in late S phase. cdc24+ encodes a novel protein with no obvious homologs in the genome databases, though the homolog exists in the related fission yeasts (S. octosporus, S. japonicus, and S. cryophilus). The fission yeast mutant, cdc24-M38, was originally identified in the cell division cycle (cdc) mutant screen thirty years ago by Kim Nasmyth and Paul Nurse as a mutant defective in the DNA synthesis phase, or S phase, of the cell cycle (Nasmyth and Nurse, 1981). Loss of cdc24 function results in [1] the arrest of the mutant cells with a single nucleus and an apparently replicated genome, and [2] chromosome breakage, which is uncharacteristic of S phase mutants (Gould et al., 1998).

Overall, our approach to defining the role of Cdc24p in genome maintenance utilizes genetics, molecular biology, cell biology, and biochemistry approaches. Preliminary work has indicated that this cdc24 mutant has genetic interactions with a group of proteins involved not only with DNA replication but also with DNA repair. We have identified several suppressors of the cdc24 temperature sensitive growth phenotype, including the fission yeast gene, dna2+, a DNA helicase/endonuclease (Kang et al., 2000). These interactions taken together with the mutant cdc24 phenotype present the exciting possibility that this gene may have a functional significance in the development of diseases of genome stability. cdc24+ is a critical gene to characterize further because it encodes a novel protein that is essential for viability and clearly interacts genetically with conserved genes required for S phase progression.

Kang, Ho-Young, Eunjoo Choi, Sung-Ho Bae, Kyoung-Hwa Lee, Byung-Soo Gim, Hee-Dai Kim, Chankyu Park, Stuart A. MacNeill, and Yeon-Soo Seo. “Genetic Analyses of Schizosaccharomyces pombe dna2+ Reveal That Dna2 Plays an Essential Role in Okazaki Fragment Metabolism.” Genetics 155, no. 3 (July 1, 2000): 1055-1067.

Nasmyth, Kim, and Paul Nurse. “Cell Division Cycle Mutants Altered in DNA Replication and Mitosis in the Fission Yeast Schizosaccharomyces pombe.” Molecular and General Genetics MGG 182, no. 1 (May 1, 1981): 119–24.

Gould, Kathleen L., C. Geoffrey Burns, Anna Feoktistova, Ching-Pei Hu, Sally G. Pasion, and Susan L. Forsburg. “Fission Yeast cdc24+ Encodes a Novel Replication Factor Required for Chromosome Integrity.” Genetics 149, no. 3 (July 1, 1998): 1221-1233.

Tanaka, Hiroyuki, Koichi Tanaka, Hiroshi Murakami, and Hiroto Okayama. “Fission Yeast Cdc24 Is a Replication Factor C- and Proliferating Cell Nuclear Antigen-Interacting Factor Essential for S-Phase Completion.” Molecular and Cellular Biology 19, no. 2 (February 1, 1999): 1038.

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