The HIRA Complex Subunit Hip3 Plays Important Roles in the Silencing of Meiosis-Specific Genes in Schizosaccharomyces pombe

BACKGROUND: The control of gene expression is essential for growth and responses to environmental changes in various organisms. It is known that some meiosis-specific genes are silenced during mitosis and expressed upon nitrogen starvation in Schizosaccharomyces pombe. When the factors responsible for this regulation were studied, a hip3 mutant was isolated via discovery of a defect in the transcriptional repression of meiosis-specific genes. Hip3 is a subunit of the HIRA (histone regulatory complex A) complex, which consists of four subunits (Hip1, Hip3, Hip4 and Slm9) and acts as a histone chaperone that is independent of DNA replication. METHODOLOGY/PRINCIPAL FINDINGS: In a search for mutants, the meiosis-specific gene SPCC663.14c (+) was identified by screening for genes that are silenced during mitosis and induced upon nitrogen starvation. A reporter plasmid that expresses the ura4 (+) gene driven by the SPCC663.14c (+) promoter was constructed. Screening for suppressor mutants was then carried out in nitrogen-rich medium without uracil. A mutant with a mutation in the hip3(+) gene was isolated and named hip3-1. This mutation alleviated the transcriptional repression of the ura4(+) gene on the reporter plasmid and of the endogenous SPCC663.14c (+) gene in the presence of nitrogen. A ChIP assay revealed that RNA polymerase II (Pol II) and TFIIE were enriched at the SPCC663.14c (+) locus, whereas the levels of histone H3 were decreased in hip3-1 cells. Intriguingly, histone H3 was heavily modified at the SPCC663.14c (+) locus in hip3-1 cells; these modifications included tri-methylation and acetylation of H3 lysine 9 (H3K9), mono-methylation of H3 arginine 2 (H3R2), and tri-methylation of H3 lysine 4 (H3K4). In addition, the tri-methylation of H3K9 and H3K4 were strongly elevated in hip3-1 mutants. CONCLUSIONS: Taken together, these results indicate that Hip3 plays important roles in the control of histone modifications at meiosis-specific gene loci and induces their transcriptional repression.