Chromatin Determinants of the Eukaryotic DNA Replication Program
The accurate and timely replication of eukaryotic DNA during S-phase is of critical importance for the cell and for the inheritance of genetic information. Missteps in the replication program can activate cell cycle checkpoints or, worse, trigger the genomic instability and aneuploidy associated with diseases such as cancer. Eukaryotic DNA replication initiates asynchronously from hundreds to tens of thousands of replication origins spread across the genome. The origins are acted upon independently, but patterns emerge in the form of large-scale replication timing domains. Each of these origins must be localized, and the activation time determined by a system of signals that, though they have yet to be fully understood, are not dependent on the primary DNA sequence. This regulation of DNA replication has been shown to be extremely plastic, changing to fit the needs of cells in development or effected by replication stress.
We have investigated the role of chromatin in specifying the eukaryotic DNA replication program. Chromatin elements, including histone variants, histone modifications and nucleosome positioning, are an attractive candidate for DNA replication control, as they are not specified fully by sequence, and they can be modified to fit the unique needs of a cell without altering the DNA template. The origin recognition complex (ORC) specifies replication origin location by binding the DNA of origins. The <italic>S. cerevisiae</italic> ORC recognizes the ARS (autonomously replicating sequence) consensus sequence (ACS), but only a subset of potential genomic sites are bound, suggesting other chromosomal features influence ORC binding. Using high-throughput sequencing to map ORC binding and nucleosome positioning, we show that yeast origins are characterized by an asymmetric pattern of positioned nucleosomes flanking the ACS. The origin sequences are sufficient to maintain a nucleosome-free origin; however, ORC is required for the precise positioning of nucleosomes flanking the origin. These findings identify local nucleosomes as an important determinant for origin selection and function. Next, we describe the <italic>D. melanogaster</italic> replication program in the context of the chromatin and transcription landscape for multiple cell lines using data generated by the modENCODE consortium. We find that while the cell lines exhibit similar replication programs, there are numerous cell line-specific differences that correlate with changes in the chromatin architecture. We identify chromatin features that are associated with replication timing, early origin usage, and ORC binding. Primary sequence, activating chromatin marks, and DNA-binding proteins (including chromatin remodelers) contribute in an additive manner to specify ORC-binding sites. We also generate accurate and predictive models from the chromatin data to describe origin usage and strength between cell lines. Multiple activating chromatin modifications contribute to the function and relative strength of replication origins, suggesting that the chromatin environment does not regulate origins of replication as a simple binary switch, but rather acts as a tunable rheostat to regulate replication initiation events.
Taken together our data and analyses imply that the chromatin contains sufficient information to direct the DNA replication program.
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