| Summary: | HIRA proteins are members of an evolutionarily conserved family of WD repeat containing histone chaperones that have been implicated in nucleosome assembly. They are known to function in multi-subunit complexes and accordingly the S. pombe HIRA proteins, Hip1 and Slm9, interact with a structurally unrelated protein called Hip3. This study has identified and characterised a fourth component of the HIRA complex, which is encoded by SPBC947.08c and has been named Hip4. Hip4 is homologous to S. cerevisiae Hpc2 and eo- immunoprecipitates with each of the previously identified HIRA complex subunits. Deletion of hip4+ leads to defects that are associated with inactivation of the HIRA complex. These include temperature sensitivity, hypersensitivity to spindle damage and defective silencing at pericentric regions and at the mat locus. Thus Hip4 is required for the integrity of heterochromatin. Consistent with this, loss of the HIRA complex leads to the accumulation of transcripts derived from centromeric repeats. However, HIRA function is not restricted to heterochromatic regions as microarray analysis revealed increased expression of at least 4% of fission yeast genes in HIRA mutants. HIRA-regulated genes overlap with targets of the Clr6 histone deacetylase, the silenced genes located in subtelomeric regions and the Tf2 long terminal repeat retrotransposons. The HIRA complex is also required for the suppression of cryptic antisense transcripts at a number of loci. Furthermore HIRA is essential in the absence of the nuclear exosome, which degrades illegitimate transcripts, consistent with a widespread role for HIRA in the suppression of spurious transcription. These findings suggest that HIRA restricts access to the genome, and consistent with this, the chromosomes of HIRA complex mutants exhibit increased susceptibility to bleomycin induced breakage. Analysis of the relationships between HIRA and other histone chaperones revealed that HIRA is essential in the absence of Chromatin Assembly Factor 1 (CAF-l). Furthermore these experiments revealed a role for the s. pombe homologue of Asfl, CialAsf1, in the suppression of antisense transcription. Therefore HIRA and CialAsf1 may cooperate to maintain nucleosomes in transcribed regions.
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