The linker histone homolog Hho1p from Saccharomyces cerevisiae represents a winged helix-turn-helix fold as determined by NMR spectroscopy

Hho1p is assumed to serve as a linker histone in Saccharomyces cerevisiae and, notably, it possesses two putative globular domains, designated HD1 (residues 41–118) and HD2 (residues 171–252), that are homologous to histone H5 from chicken erythrocytes. We have determined the three-dimensional structure of globular domain HD1 with high precision by heteronuclear magnetic resonance spectroscopy. The structure had a winged helix–turn–helix motif composed of an αβααββ fold and closely resembled the structure of the globular domain of histone H5. Interestingly, the second globular domain, HD2, in Hho1p was unstructured under physiological conditions. Gel mobility assay demonstrated that Hho1p preferentially binds to supercoiled DNA over linearized DNA. Furthermore, NMR analysis of the complex of a deletion mutant protein (residues 1–118) of Hho1p with a linear DNA duplex revealed that four regions within the globular domain HD1 are involved in the DNA binding. The above results suggested that Hho1p possesses properties similar to those of linker histones in higher eukaryotes in terms of the structure and binding preference towards supercoiled DNA.     

Distinct properties of the two putative "globular domains" of the yeast linker histone, Hho1p

The putative linker histone in Saccharomyces cerevisiae, Hho1p, has two regions of sequence (GI and GII) that are homologous to the single globular domains of linker histones H1 and H5 in higher eukaryotes. However, the two Hho1p "domains" differ with respect to the conservation of basic residues corresponding to the two putative DNA-binding sites (sites I and II) on opposite faces of the H5 globular domain. We find that GI can protect chromatosome-length DNA, like the globular domains of H1 and H5 (GH1 and GH5), but GII does not protect. However, GII, like GH1 and GH5, binds preferentially (and with higher affinity than GI) to four-way DNA junctions in the presence of excess linear DNA competitor, and binds more tightly than GI to linker-histone-depleted chromatin. Surprisingly, in 10 mM sodium phosphate (pH 7.0), GII is largely unfolded, whereas GI, like GH1 and GH5, is structured, with a high alpha-helical content. However, in the presence of high concentrations of large tetrahedral anions (phosphate, sulphate, perchlorate) GII is also folded; the anions presumably mimic DNA in screening the positive charge. This raises the possibility that chromatin-bound Hho1p may be bifunctional, with two folded nucleosome-binding domains.     

Homology model building of Hho1p supports its role as a yeast histone H1 protein

Biochemical studies to date have not been able to identify the linker histone H1 protein in the budding yeast Saccharomyces cerevisiae. Database homology searching against the complete yeast genome has identified a gene, HHO1, (or YPL127C, formerly LPI17) which encodes a protein that has two regions that show similarity to the pea histone H1 globular domain. To determine whether Hho1p can assume the shape of an H1 protein, homology model building experiments were performed using the structure of chicken histone H5 globular domain as the basis for comparison. A statistically significant match between each of the two globular domains of Hho1p and the chicken histone H5 structure was obtained, and probability values indicate that there is a less than 1 in 100 chance that such a match would be the result of a random event. These findings support the proposal that Hho1p acts as an "H1 dimer" and could be responsible for the decreased linker DNA length observed between nucleosomal core particles.     

Two homologous domains of similar structure but different stability in the yeast linker histone, Hho1p

The Saccharomyces cerevisiae homologue of the linker histone H1, Hho1p, has two domains that are similar in sequence to the globular domain of H1 (and variants such as H5). It is an open question whether both domains are functional and whether they play similar structural roles. Preliminary structural studies showed that the two isolated domains, GI and GII, differ significantly in stability. In 10 mM sodium phosphate (pH 7), the GI domain, like the globular domains of H1 and H5, GH1 and GH5, was stably folded, whereas GII was largely unstructured. However, at high concentrations of large tetrahedral anions (phosphate, sulphate, perchlorate), which might mimic the charge-screening effects of DNA phosphate groups, GII was folded. In view of the potential significance of these observations in relation to the role of Hho1p, we have now determined the structures of its GI and GII domains by NMR spectroscopy under conditions in which GII (like GI) is folded. The backbone r.m.s.d. over the ordered residues is 0.43 A for GI and 0.97 A for GII. Both structures show the "winged-helix" fold typical of GH1 and GH5 and are very similar to each other, with an r.m.s.d. over the structured regions of 1.3 A, although there are distinct differences. The potential for GII to adopt a structure similar to that of GI when Hho1p is bound to chromatin in vivo suggests that both globular domains might be functional. Whether Hho1p performs a structural role by bridging two nucleosomes remains to be determined.     

The Saccharomyces cerevisiae linker histone Hho1p, with two globular domains, can simultaneously bind to two four-way junction DNA molecules

Saccharomyces cerevisiae encodes a single linker histone, Hho1p, with two globular domains. This raised the possibility that Hho1p could bind to two nucleosome cores simultaneously. To evaluate this idea, we studied the ability of a four-way junction, immobilized on the surface of a magnetic bead, to pull down a radiolabeled four-way junction in the presence of different Hho1 proteins. Four-way junctions are known to bind to H1, presumably due to structure similarities to the DNA at the nucleosomal entry/exit point. We found a significant increase in the ability of full-length Hho1p to pull down radiolabeled four-way junction DNA under ionic conditions where both globular domains could bind. The binding was structure specific, since the use of double-stranded DNA, or a mutant Hho1p in which the second DNA binding site of globular domain 1 was abolished, resulted in a significant decrease in bridged binding. Additionally, bridged binding required a covalent attachment between the two globular domains, since factor Xa protease treatment of the complex formed by a modified Hho1p that contained a factor Xa cleavage site between the two globular domains resulted in a significant release of radiolabeled four-way junction. These findings demonstrated that the two globular domains independently associated with two different four-way junction molecules in a manner that required amino acid residues implicated in structure-specific binding in the nucleosome. We discuss the implication of these findings on the chromatin structure of yeast and propose a model where a single Hho1 protein binds to two serially adjacent nucleosomes.     

Hho1p, the linker histone of Saccharomyces cerevisiae, is important for the proper chromatin organization in vivo

Despite the existence of certain differences between yeast and higher eukaryotic cells a considerable part of our knowledge on chromatin structure and function has been obtained by experimenting on Saccharomyces cerevisiae. One of the peculiarities of S. cerevisiae cells is the unusual and less abundant linker histone, Hho1p. Sparse is the information about Hho1p involvement in yeast higher-order chromatin organization. In an attempt to search for possible effects of Hho1p on the global organization of chromatin, we have applied Chromatin Comet Assay (ChCA) on HHO1 knock-out yeast cells. The results showed that the mutant cells exhibited highly distorted higher-order chromatin organization. Characteristically, linker histone depleted chromatin generally exhibited longer chromatin loops than the wild-type. According to the Atomic force microscopy data the wild-type chromatin appeared well organized in structures resembling quite a lot the "30-nm" fiber in contrast to HHO1 knock-out yeast.     

Hho1p,the linker histone of Saccharomyces cerevisiae, is important for the proper chromatin organization in vivo

Despite the existence of certain differences between yeast and higher eukaryotic cells a considerable part of our knowledge on chromatin structure and function has been obtained by experimenting on Saccharomyces cerevisiae. One of the peculiarities of S. cerevisiae cells is the unusual and less abundant linker histone, Hho1p. Sparse is the information about Hho1p involvement in yeast higher-order chromatin organization. In an attempt to search for possible effects of Hho1p on the global organization of chromatin, we have applied Chromatin Comet Assay (ChCA) on HHO1 knock-out yeast cells. The results showed that the mutant cells exhibited highly distorted higher-order chromatin organization. Characteristically, linker histone depleted chromatin generally exhibited longer chromatin loops than the wild-type. According to the Atomic force microscopy data the wild-type chromatin appeared well organized in structures resembling quite a lot the “30-nm” fiber in contrast to HHO1 knock-out yeast.     

Control of DNA end resection by yeast Hmo1p affects efficiency of DNA end-joining

The primary pathways for DNA double strand break (DSB) repair are homologous recombination (HR) and non-homologous end–joining (NHEJ). The choice between HR and NHEJ is influenced by the extent of DNA end resection, as extensive resection is required for HR but repressive to NHEJ. Conversely, association of the DNA end-binding protein Ku, which is integral to classical NHEJ, inhibits resection. In absence of key NHEJ components, a third repair pathway is exposed; this alternative-end joining (A-EJ) is a highly error-prone process that uses micro-homologies at the breakpoints and is initiated by DNA end resection. In Saccharomyces cerevisiae, the high mobility group protein Hmo1p has been implicated in controlling DNA end resection, suggesting its potential role in repair pathway choice. Using a plasmid end-joining assay, we show here that absence of Hmo1p results in reduced repair efficiency and accuracy, indicating that Hmo1p promotes end-joining; this effect is only observed on DNA with protruding ends. Notably, inhibition of DNA end resection in an hmo1Δ strain restores repair efficiency to the levels observed in wild-type cells. In absence of Ku, HMO1 deletion also reduces repair efficiency further, while inhibition of resection restores repair efficiency to the levels observed in kuΔ. We suggest that Hmo1p functions to control DNA end resection, thereby preventing error-prone A-EJ repair and directing repairs towards classical NHEJ. The very low efficiency of DSB repair in kuΔhmo1Δ cells further suggests that excessive DNA resection is inhibitory for A-EJ.     

NAP1 family histone chaperones are required for somatic homologous recombination in Arabidopsis

Homologous recombination (HR) is essential for maintaining genome integrity and variability. To orchestrate HR in the context of chromatin is a challenge, both in terms of DNA accessibility and restoration of chromatin organization after DNA repair. Histone chaperones function in nucleosome assembly/disassembly and could play a role in HR. Here, we show that the NUCLEOSOME ASSEMBLY PROTEIN1 (NAP1) family histone chaperones are required for somatic HR in Arabidopsis thaliana. Depletion of either the NAP1 group or NAP1-RELATED PROTEIN (NRP) group proteins caused a reduction in HR in plants under normal growth conditions as well as under a wide range of genotoxic or abiotic stresses. This contrasts with the hyperrecombinogenic phenotype caused by the depletion of the CHROMATIN ASSEMBLY FACTOR-1 (CAF-1) histone chaperone. Furthermore, we show that the hyperrecombinogenic phenotype caused by CAF-1 depletion relies on NRP1 and NRP2, but the telomere shortening phenotype does not. Our analysis of DNA lesions, H3K56 acetylation, and expression of DNA repair genes argues for a role of NAP1 family histone chaperones in nucleosome disassembly/reassembly during HR. Our study highlights distinct functions for different families of histone chaperones in the maintenance of genome stability and establishes a crucial function for NAP1 family histone chaperones in somatic HR.     

Chromodomain helicase DNA-binding protein 4 (CHD4) regulates homologous recombination DNA repair, and its deficiency sensitizes cells to poly(ADP-ribose) polymerase (PARP) inhibitor treatment

To ensure genome stability, cells have evolved a robust defense mechanism to detect, signal, and repair damaged DNA that is generated by exogenous stressors such as ionizing radiation, endogenous stressors such as free radicals, or normal physiological processes such as DNA replication. Homologous recombination (HR) repair is a critical pathway of repairing DNA double strand breaks, and it plays an essential role in maintaining genomic integrity. Previous studies have shown that BRIT1, also known as MCPH1, is a key regulator of HR repair. Here, we report that chromodomain helicase DNA-binding protein 4 (CHD4) is a novel BRIT1 binding partner that regulates the HR repair process. The BRCA1 C-terminal domains of BRIT1 are required for its interaction with CHD4. Depletion of CHD4 and overexpression of the ATPase-dead form of CHD4 impairs the recruitment of BRIT1 to the DNA damage lesions. As a functional consequence, CHD4 deficiency sensitizes cells to double strand break-inducing agents, reduces the recruitment of HR repair factor BRCA1, and impairs HR repair efficiency. We further demonstrate that CHD4-depleted cells are more sensitive to poly(ADP-ribose) polymerase inhibitor treatment. In response to DNA damage induced by poly(ADP-ribose) polymerase inhibitors, CHD4 deficiency impairs the recruitment of DNA repair proteins BRIT1, BRCA1, and replication protein A at early steps of HR repair. Taken together, our findings identify an important role of CHD4 in controlling HR repair to maintain genome stability and establish the potential therapeutic implications of targeting CHD4 deficiency in tumors.     

Dissecting the role of p53 phosphorylation in homologous recombination provides new clues for gain-of-function mutants

Regulation of homologous recombination (HR) represents the best-characterized DNA repair function of p53. The role of p53 phosphorylation in DNA repair is largely unknown. Here, we show that wild-type p53 repressed repair of DNA double-strand breaks (DSBs) by HR in a manner partially requiring the ATM/ATR phosphorylation site, serine 15. Cdk-mediated phosphorylation of serine 315 was dispensable for this anti-recombinogenic effect. However, without targeted cleavage of the HR substrate, serine 315 phosphorylation was necessary for the activation of topoisomerase I-dependent HR by p53. Moreover, overexpression of cyclin A1, which mimics the situation in tumors, inappropriately stimulated DSB-induced HR in the presence of oncogenic p53 mutants (not Wtp53). This effect required cyclin A1/cdk-mediated phosphorylation for stable complex formation with topoisomerase I. We conclude that p53 mutants have lost the balance between activation and repression of HR, which results in a net increase of potentially mutagenic DNA rearrangements. Our data provide new insight into the mechanism underlying gain-of-function of mutant p53 in genomic instability.