The effect of histone H3 and H4 mutants on the chronological lifespan of Saccharomyces cerevisiae
In this study we maintained histone mutant strains in a batch culture. We observed that some strains significantly decreased at a lower rate in the quiescent population.Some strains disappeared from quiescent population more rapidly, surprisingly, these strains did not increase in the non-quiescent population, suggesting that they may lyse and die.Residues that are implicated in lifespan extension are mostly histone tail residues and residues that are located in the solvent accessible region, suggesting that these residues may be bound by another protein(s) such as the silent information regulator, Sir3, via a BAH domain.All residues that are implicated in lifespan reduction are localised internally on the nucleosome core except for H3K18, suggesting their role in destabilising the nucleosome structure. This may allow elevated levels of DNA damage, and induce apoptosis. The gene expression studies of cells at late log phaseshowed that although all selected strains exhibited chronological lifespan extension, they had two distinct regulatory processes, suggesting that the H4K16Q and H4H18A strains, on the one hand, and the H3E50 strain on the other hand, may modulate different pathways to regulate chronological lifespan.The GO term enrichment for genes that are significantly up-regulated in the H4K16Q and H4H18A mutants, include rRNA processing and maturation, rRNA export, ribosome assembly, gene expression, cytoplasmic translation and ethanol metabolic processes. On the other hand, the GO term enrichment for induced genes in H3E50A mutant, include, mitochondrial electron transport, tricarboxylic acid cycle, ATP synthesis coupled proton transport, oxidative phosphorylation, glutamate metabolic process, glyoxylate cycle, and purine ribonucleoside triphosphate biosynthetic process. In addition, response to hydrogen peroxide, reactive oxygen species metabolic process, and response to oxidative stress appeared. It appears that many metabolic processes are upregulated in the H3E50A mutant strain. It is likely that the increased metabolism leads to storage of energy to be used in long starvation periods. The proteome studies of H4K16Q and H4H18A mutants at late stages of growth showed similar profiles. Snz1 is involved in vitamin B6 biosynthesis. It is regulated by, among others, Rap1. Jhd2 induction is interesting because it is a histone demethylase involved in global demethylation of H3K4, a histone mark added by Set1 and associated with active transcription. Gcy1 is a glycerol dehydrogenase, and was shown to increase in response to DNA replication stress. In H3E50A mutant, ribosomal protein, RPL16B, which is regulated by Rap1 was upregulated. Scs2 is involved in phospholipid metabolism, it was also shown that Scs2 over-expression rescued telomeric shortening and was de-repressed in a strain over-expressing Mec1. Tfs1 is an inhibitor of carboxypeptidase Y, and was also shown to regulate the PKA signalling pathway. We propose that Sir3, which is recruited to the silent chromatin by Rap1, cannot bind to the telomere nucleosomes, therefore, Rap1 redistributes to other regions of the genome of histone mutants. The regions, which Rap1 may relocate to, include stress response, DNA replication and damage stress response, and telomere maintenance genes.