Yeast Mrc1, ortholog of metazoan Claspin, is both a central component of normal DNA replication forks and a mediator of the S phase checkpoint. We report that Mrc1 interacts with Pol2, the catalytic subunit of DNA polymerase e, essential for leading strand DNA replication and for the checkpoint. In unperturbed cells, Mrc1 interacts independently with both the N-terminal and C-terminal halves of Pol2 (Pol2N and Pol2C). Strikingly, phosphorylation of Mrc1 during the S phase checkpoint abolishes Pol2N binding but not Pol2C interaction. Mrc1 is required to stabilize Pol2 at replication forks stalled in HU. The bimodal Mrc1/Pol2 interaction may identify a novel step in regulating the S phase checkpoint response to DNA damage on the leading strand. We propose that Mrc1, which also interacts with the MCMs, may modulate coupling of polymerization and unwinding at the replication fork.
Overall design: Type of experiment: ChIP-chip (Chromatin immunoprecipitation on DNA chip) analyses of replication related, checkpoint related proteins and BrdU incorporated regions at 300bp resolution. S.cerevisiae chromosome VI tiling array produced by affymetrix was used.
Experimental factors:Distribution of DNA Pol2 (epsilon) on chromosome VI during S-phase (with hydroxy urea). Wild type, tof1 deletion mutant, and mrc1 deletion mutant were investigated.
The type of reference used for the hybridizations, if any: For the analysis of HA tagged Pol2 protein, POL1-3XFLAG was analyzed simultaneously as a reference.
Hybridization design: Comparison of ChIP fraction with SUP fraction for protein binding profile. No locus was scored as significantly enriched in the control ChIP experiment using HU treated untagged cells.
Quality control steps taken: Confirmation of ChIP fraction by Western blotting. Confirmation by conventional ChIP PCR methods for selected locus.
Manipulation of biological samples and protocols used:For the preparation of HU-arrested cells, we synchronized yeast cells with alpha-factor (2mM) and then released cells into 200mM HU for 60 min. at 23°C. Cells were fixed by 1% Formaldehyde and then used for ChIP of protein of interest.
Protocol for preparing the hybridization extract:We disrupted 1.5X10^8 cells by MULTI-BEADS SHOCKER (MB400U, YASUI KIKAI, Osaka), which was able to keep cells precisely at lower than 6°C during disruption by glass beads. This enabled us to retrieve more than 60% of tagged proteins to soluble fraction routinely. Chromosomal DNA was shared into the average size of 400-600bp by sonication. Anti-HA monoclonal antibody HA.11 (16B12) (CRP Inc., Denver, PA) and anti-FLAG monoclonal antibody M2 (Sigma-Aldrich Co., St Louis, MO) were used for chromatin immuno-precipitation. Immunoprecipitated chromosomal DNA was subsequently purified and amplified by PCR. 2ug of amplified DNA was digested with DNaseI to a mean size of 100bp and used for labeling as previously described by Winzeler et al. (Science. 281, 1194-1197, 1998).
Labeling protocol:DNA was end-labelled with Biotin-N6ddATP by Terminal Transferase as previously described by Winzeler et al. (Science. 281, 1194-1197, 1998)
The protocol and conditions used during hybridization, blocking and washing: Hybridization, blocking and washing steps were carried out as previously described by Winzeler et al. (Science. 281, 1194-1197, 1998). Each sample was hybridized to the array in 150ul containing 6XSSPE, 0.005% TritonX-100, 15ug fragmented denatured salmon sperm DNA (Gibco-BRL), and 1nmole of a 3’-biotin control oligonucleotide (oligo B2 probes) that hybridized to the border features on the array. Samples were heated to 100°C for 10 min., and then cooled on ice. Samples were hybridized for 16h at 42°C in a hybridization oven (GeneChip hybri oven 320, Affymetrix, CA). Washing and staining protocol (Mini_euk1 ver2) provided by Affymetrix was performed automatically on a fluidics station (GeneChip fluidics station 400, Affymetrix, CA).
Measurement data and specifications:Each array was scanned by HP GeneArray Scanner (Affymetrix, CA) at an emission wavelength of 560nm at 7.5uM resolution. Grids were aligned to the scanned images using the know feature of the array. Primary data analyses were carried out using the Affymetrix Microarray Suite Ver.5.0 software to obtain hybridization intensity, fold change value, change p-value, and detection p-value for each locus. For the discrimination of positive and negative signals for the binding, we compared ChIP fraction with Sup (supernatant) fraction by using three criteria as follows. First, the reliability of strength of signal was judged by detection p-value of each locus (p-value lower than 0.025 was considered as significant). Secondly, reliability of binding ratio was judged by change p-value (p-value lower than 0.025 was considered as significant). Thirdly, clusters consisted of at least three contiguous loci which filled above two criteria were selected as significantly enriched locus, because it is apparent that a single site of protein-DNA interaction will result in immuno-precipitation of DNA fragments that hybridized not only to the locus of the actual binding site but also to its neighbors. This third criterion is very unique, make the data highly reliable and can be only applicable to the chip data obtained by our high-resolution tiling array.
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