Crystallization and preliminary X-ray characterization of the Methanothermus fervidus histones HMfA and HMfB

The Role of Archaeal Chromatin in Transcription

Genomic organization impacts accessibility and movement of information processing systems along DNA. DNA-bound proteins dynamically dictate gene expression and provide regulatory potential to tune transcription rates to match ever-changing environmental conditions. Archaeal genomes are typically small, circular, gene dense, and organized either by histone proteins that are homologous to their eukaryotic counterparts, or small basic proteins that function analogously to bacterial nucleoid proteins. We review here how archaeal genomes are organized and how such organization impacts archaeal gene expression, focusing on conserved DNA-binding proteins within the clade and the factors that are known to impact transcription initiation and elongation within protein-bound genomes.

The hydrophobicity of the H3 histone fold differs from the hydrophobicity of the other three folds

The eukaryotic histone dimers, H3-H4 and H2A-H2B, are formed in the cytosol prior to being transported into the nucleus and assembled into the nucleosome. Residue side-chain distances from the interior of the histone dimers are obtained with an ellipsoidal spatial metric and structural information provided by X-ray analyses at atomic resolution of the nucleosome core particles. While the spatial hydrophobic moment profiles of the dimers are comparable with profiles obtained previously that characterize the hydrophobic core of single-chain, single-domain globular soluble proteins, correlation coefficients between the side-chain hydrophobicities and distances from the interior of the H3-H4 dimer and H2A-H2B dimer differ significantly. This difference is traced to the H3 histone fold, which segregates fewer hydrophobic residues within the protein interior than the three other folds. Examination of the correlation coefficient between residue hydrophobicity and side-chain distance from the dimer interior over local regions of the fold sequence shows that the region of reduced correlation is associated mainly with the residues at the carboxyl end of the H3 histone fold, the helical region of the fold involved in the H3-H3' binding of the (H3-H4)(2) tetramer of the nucleosome. Hydrophobic interactions apparently contribute to the binding of this fourfold helical bundle and this evolutionary requirement may trade off against the requirement for H3-H4 dimer stability. The present results provide a different view than previously proposed, albeit of similar origin, to account for the reduced stability of the H3-H4 dimer compared with the H2A-H2B dimer.

Crystal structures of recombinant histones HMfA and HMfB from the hyperthermophilic archaeon Methanothermus fervidus

The hyperthermophilic archaeon Methanothermus fervidus contains two small basic proteins, HMfA (68 amino acid residues) and HMfB (69 residues) that share a common ancestry with the eukaryal nucleosome core histones H2A, H2B, H3, and H4. HMfA and HMfB have sequences that differ at 11 locations, they have different structural stabilities, and the complexes that they form with DNA have different electrophoretic mobilities. Here, crystal structures are documented for recombinant (r) HMfA at a resolution of 1.55 A refined to a crystallographic R-value of 19.8 % (tetragonal form) and at 1.48 A refined to a R-value of 18.8 % (orthorhombic form), and for rHMfB at 1.9 A refined to a R-value of 18.0 %. The rHMfA and rHMfB monomers have structures that are just histone folds in which a long central alpha-helix (alpha2; 29 residues) is separated from shorter N-terminal (alpha1; 11 residues) and C-terminal (alpha3; 10 residues) alpha-helices by two loops (L1 and L2; both 6 residues). Within L1 and L2, three adjacent residues are in extended (beta) conformation. rHMfA and rHMfB assemble into homodimers, with the alpha2 helices anti-parallel aligned and crossing at an angle of close to 35 degrees, and with hydrogen bonds formed between the extended, parallel regions of L1 and L2 resulting in short beta-ladders. Dimerization creates a novel N-terminal structure that contains four proline residues, two from each monomer. As prolines are present at these positions in all archaeal histone sequences, this proline-tetrad structure is likely to be a common feature of all archaeal histone dimers. Almost all residues that participate in monomer-monomer interactions are conserved in HMfA and HMfB, consistent with the ability of these monomers to form both homodimers and (HMfA+HMfB) heterodimers. Differences in side-chain interactions that result from non-conservative residue differences in HMfA and HMfB are identified, and the structure of a (rHMfA)(2)-DNA complex is presented based on the structures documented here and modeled by homology to histone-DNA interactions in the eukaryal nucleosome.

Mutational analysis of differences in thermostability between histones from mesophilic and hyperthermophilic archaea

Amino acid residues responsible for the large difference in thermostability between HMfB and HFoB, archaeal histones from the hyperthermophile Methanothermus fervidus and the mesophile Methanobacterium formicicum, respectively, have been identified by site-specific mutagenesis. The thermal denaturation of approximately 70 archaeal histone variants has been monitored by circular dichroism, and the data generated were fit to a two-state unfolding model (dimer-->two random coil monomers) to obtain a standard-state (1M) melting temperature for each variant dimer. The results of single-, double-, and triple-residue substitutions reveal that the much higher stability of rHMfB dimers, relative to rHFoB dimers, is conferred predominantly by improved intermolecular hydrophobic interactions near the center of the histone dimer core and by additional favorable ion pairs on the dimer surface.

Archaeal nucleosome positioning sequence from Methanothermus fervidus

DNA in Methanothermus fervidus, a hyperthermophilic archaeon, is constrained into archaeal nucleosomes in vivo by the archaeal histones HMfA and HMfB. Here, we document the translational and rotational positioning of archaeal nucleosome assembly in vitro by a sequence from the 7S RNA encoding region of the M. fervidus genome. The minor groove of the DNA at the center of the DNA sequence, protected from micrococcal nuclease digestion by incorporation into a positioned archaeal nucleosome, faces away from the archaeal histone core.