DNA-Binding Proteins and Chromatin. In: Cavicchioli R, editor. Archaea. Washington: ASM Press; 2007. pp. 110-9. [DOI: 10.1128/9781555815516.ch4]

Archaeal nucleosome positioning in vivo and in vitro is directed by primary sequence motifs

Background

Histone wrapping of DNA into nucleosomes almost certainly evolved in the Archaea, and predates Eukaryotes. In Eukaryotes, nucleosome positioning plays a central role in regulating gene expression and is directed by primary sequence motifs that together form a nucleosome positioning code. The experiments reported were undertaken to determine if archaeal histone assembly conforms to the nucleosome positioning code.

Results

Eukaryotic nucleosome positioning is favored and directed by phased helical repeats of AA/TT/AT/TA and CC/GG/CG/GC dinucleotides, and disfavored by longer AT-rich oligonucleotides. Deep sequencing of genomic DNA protected from micrococcal nuclease digestion by assembly into archaeal nucleosomes has established that archaeal nucleosome assembly is also directed and positioned by these sequence motifs, both in vivo in Methanothermobacter thermautotrophicus and Thermococcus kodakarensis and in vitro in reaction mixtures containing only one purified archaeal histone and genomic DNA. Archaeal nucleosomes assembled at the same locations in vivo and in vitro, with much reduced assembly immediately upstream of open reading frames and throughout the ribosomal rDNA operons. Providing further support for a common positioning code, archaeal histones assembled into nucleosomes on eukaryotic DNA and eukaryotic histones into nucleosomes on archaeal DNA at the same locations. T. kodakarensis has two histones, designated HTkA and HTkB, and strains with either but not both histones deleted grow normally but do exhibit transcriptome differences. Comparisons of the archaeal nucleosome profiles in the intergenic regions immediately upstream of genes that exhibited increased or decreased transcription in the absence of HTkA or HTkB revealed substantial differences but no consistent pattern of changes that would correlate directly with archaeal nucleosome positioning inhibiting or stimulating transcription.

Conclusions

The results obtained establish that an archaeal histone and a genome sequence together are sufficient to determine where archaeal nucleosomes preferentially assemble and where they avoid assembly. We confirm that the same nucleosome positioning code operates in Archaea as in Eukaryotes and presumably therefore evolved with the histone-fold mechanism of DNA binding and compaction early in the archaeal lineage, before the divergence of Eukaryotes.

Global proteomic analysis of the insoluble, soluble, and supernatant fractions of the psychrophilic archaeon Methanococcoides burtonii. Part I: the effect of growth temperature

The response of the cold-adapted (psychrophilic) methanogenic archaeon Methanococcoides burtonii to growth temperature was investigated using differential proteomics (postincorporation isobaric labeling) and tandem liquid chromatography-mass spectrometry (LC/LC-MS/MS). This is the first proteomic study of M. burtonii to include techniques that specifically enrich for both surface and membrane proteins and to assess the effects of growth temperature (4 vs 23 degrees C) and carbon source (trimethylamine vs methanol) on cellular protein levels. Numerous surface layer proteins were more abundant at 4 degrees C, indicating an extensive remodeling of the cell envelope in response to low temperature. Many of these surface proteins contain domains associated with cell adhesion. Within the cell, small proteins each composed of a single TRAM domain were recovered as important cold adaptation proteins and might serve as RNA chaperones, in an analogous manner to Csp proteins (absent from M. burtonii). Other proteins that had higher abundances at 4 degrees C can be similarly tied to relieving or resolving the adverse affects of cold growth temperature on translational capacity and correct protein folding. The proteome of M. burtonii grown at 23 degrees C was dominated by oxidative stress proteins, as well as a large number of integral membrane proteins of unknown function. This is the first truly global proteomic study of a psychrophilic archaeon and greatly expands knowledge of the cellular mechanisms underpinning cold adaptation in the Archaea.

Archaeal chromatin proteins histone HMtB and Alba have lost DNA-binding ability in laboratory strains of Methanothermobacter thermautotrophicus

Alignments of the sequences of the all members of the archaeal histone and Alba1 families of chromatin proteins identified isoleucine residues, I19 in HMtB and I39 in MtAlba, in Methanothermobacter thermautotrophicus, at locations predicted to be directly involved in DNA binding. In all other HMfB family members, residue 19 is an arginine (R19), and either arginine or lysine is present in almost all other Alba1 family members at the structural site equivalent to I39 in MtAlba. Electrophoretic mobility shift assays revealed that recombinant HMtB and MtAlba do not bind DNA, but variants constructed with R19 and R39, respectively, bound DNA; and whereas MtAlba(I19) did not bind RNA, MtAlba(R19) bound both single stranded RNA and tRNA. Amplification and sequencing of MT0254 (encodes HMtB) and MT1483 (encodes MtAlba) from several Methanothermobacter thermautotrophicus lineages has revealed that HMtB and MtAlba had arginine residues at positions 19 and 39, respectively, in the original isolate and that spontaneous mutations must have occurred, and been fixed, in some laboratory lineages that now have HMtB(I19) and MtAlba(I39). The retention of these variants suggests some continuing functions and fusion of the HMtB(I19) sequence to HMtA2 resulted in a protein that folds to form a histone fold heterodimer that binds and compacts DNA. The loss of DNA binding by HMtB(I19) does not therefore prevent HMtB from participating in DNA interactions as one partner of an archaeal histone heterodimer.