Abstract
Faithful DNA replication is a prerequisite for cell proliferation. Several cytological studies have shown that chromosome structures alter in the S-phase of the cell cycle. However, the molecular mechanisms behind the alteration of chromosome structures associated with DNA replication have not been elucidated. Here, we investigated chromatin structures and acetylation of specific histone residues during DNA replication using the meiotic nucleus of the fission yeast Schizosaccharomyces pombe. The S. pombe meiotic nucleus provides a unique opportunity for measuring the levels of compaction of chromatin along the chromosome in a defined orientation. By direct measurement of chromatin compaction in living cells, we demonstrated that decompaction of chromatin occurs during meiotic DNA replication. This chromatin decompaction was suppressed by depletion of histone acetyltransferase Mst1 or by arginine substitution of specific lysine residues (K8 and K12) of histone H4. These results suggest that acetylation of histone H4 residues K8 and K12 plays a critical role in loosening chromatin structures during DNA replication.
Highlights
Taking an advantage of this characteristic nuclear morphology in S. pombe meiosis, we previously isolated a csn[1] mutant in a screening of sporulation-deficient mutants that exhibits aberrant nuclear morphology[12]
A csn1-deletion strain exhibited a characteristic phenotype during nuclear movements[12] similar to that reported in rec8∆ strain[11]: the tip of the horsetail nucleus moved back and forth within the cell while the bulk of the nucleus did not follow the movements (Supplementary Fig. S1B), implying that the chromatin was stretched under the pulling forces of horsetail movement
Our direct measurements of chromatin compaction in the meiotic horsetail nucleus of living S. pombe cells demonstrated that chromatin decompaction occurs during meiotic DNA replication
Summary
Taking an advantage of this characteristic nuclear morphology in S. pombe meiosis, we previously isolated a csn[1] mutant in a screening of sporulation-deficient mutants that exhibits aberrant nuclear morphology[12]. The csn1+ gene encodes the subunit 1 of COP9 signalosome complex[13], which is required for activity of the Pcu4-Ddb1Cdt[2] E3 ubiquitin ligase complex[14,15]; COP9 signalosome complex and Pcu4-Ddb1Cdt[2] ubiquitin ligase are required for degradation of Spd[1], an inhibitor of ribonucleotide reductase (RNR). Because RNR activity is necessary to convert ribonucleotides to deoxyribonucleotides (dNTPs), deficiency of Csn[1] causes stalled DNA replication due to inefficiency of dNTP synthesis[14,15]. We measured chromatin compaction in the csn[1] mutant, and identified an association between histone H4 modification and chromatin compaction during DNA replication
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