The nucleosome repeat length of Kluyveromyces lactis is 16 bp longer than that of Saccharomyces cerevisiae

Kluyveromyces lactis genome harbours a functional linker histone encoding gene

Linker histones are essential components of chromatin in eukaryotes. Through interactions with linker DNA and nucleosomes they facilitate folding and maintenance of higher-order chromatin structures and thus delicately modulate gene activity. The necessity of linker histones in lower eukaryotes appears controversial and dubious. Genomic data have shown that Schizosaccharomyces pombe does not possess genes encoding linker histones while Kluyveromyces lactis has been reported to have a pseudogene. Regarding this controversy, we have provided the first direct experimental evidence for the existence of a functional linker histone gene, KlLH1, in K. lactis genome. Sequencing of KlLH1 from both genomic DNA and copy DNA confirmed the presence of an intact open reading frame. Transcription and splicing of the KlLH1 sequence as well as translation of its mRNA have been studied. In silico analysis revealed homology of KlLH1p to the histone H1/H5 protein family with predicted three domain structure characteristic for the linker histones of higher eukaryotes. This strongly proves that the yeast K. lactis does indeed possess a functional linker histone gene thus entailing the evolutionary preservation and significance of linker histones. The nucleotide sequences of KlLH1 are deposited in the GenBank under accession numbers KT826576, KT826577 and KT826578.

Comparative Genomics Reveals Chd1 as a Determinant of Nucleosome Spacing in Vivo

Packaging of genomic DNA into nucleosomes is nearly universally conserved in eukaryotes, and many features of the nucleosome landscape are quite conserved. Nonetheless, quantitative aspects of nucleosome packaging differ between species because, for example, the average length of linker DNA between nucleosomes can differ significantly even between closely related species. We recently showed that the difference in nucleosome spacing between two Hemiascomycete species—Saccharomyces cerevisiae and Kluyveromyces lactis—is established by trans-acting factors rather than being encoded in cis in the DNA sequence. Here, we generated several S. cerevisiae strains in which endogenous copies of candidate nucleosome spacing factors are deleted and replaced with the orthologous factors from K. lactis. We find no change in nucleosome spacing in such strains in which H1 or Isw1 complexes are swapped. In contrast, the K. lactis gene encoding the ATP-dependent remodeler Chd1 was found to direct longer internucleosomal spacing in S. cerevisiae, establishing that this remodeler is partially responsible for the relatively long internucleosomal spacing observed in K. lactis. By analyzing several chimeric proteins, we find that sequence differences that contribute to the spacing activity of this remodeler are dispersed throughout the coding sequence, but that the strongest spacing effect is linked to the understudied N-terminal end of Chd1. Taken together, our data find a role for sequence evolution of a chromatin remodeler in establishing quantitative aspects of the chromatin landscape in a species-specific manner.

Toward a unified physical model of nucleosome patterns flanking transcription start sites

Recent genome-wide maps of nucleosome positions in different eukaryotes revealed patterns around transcription start sites featuring a nucleosome-free region flanked by a periodic modulation of the nucleosome density. For Saccharomyces cerevisiae, the average in vivo pattern was previously shown to be quantitatively described by a "nucleosome gas" model based on the statistical positioning mechanism. However, this simple physical description is challenged by the fact that the pattern differs quantitatively between species and by recent experiments that appear incompatible with statistical positioning, indicating important roles for chromatin remodelers. We undertake a data-driven search for a unified physical model to describe the nucleosome patterns of 12 yeast species and also consider an extension of the model to capture remodeling effects. We are led to a nucleosome gas that takes into account nucleosome breathing, i.e., transient unwrapping of nucleosomal DNA segments. This known biophysical property of nucleosomes rationalizes a "pressure"-induced dependence of the effective nucleosome size that is suggested by the data. By fitting this model to the data, we find an average energy cost for DNA unwrapping consistent with previous biophysical experiments. Although the available data are not sufficient to reconstruct chromatin remodeling mechanisms, a minimal model extension by one mechanism yields an "active nucleosome gas" that can rationalize the behavior of systems with reduced histone-DNA ratio and remodeler knockouts. We therefore establish a basis for a physical description of nucleosome patterns that can serve as a null model for sequence-specific effects at individual genes and in models of transcription regulation.

A functional evolutionary approach to identify determinants of nucleosome positioning: a unifying model for establishing the genome-wide pattern

Although the genomic pattern of nucleosome positioning is broadly conserved, quantitative aspects vary over evolutionary timescales. We identify the cis and trans determinants of nucleosome positioning using a functional evolutionary approach involving S. cerevisiae strains containing large genomic regions from other yeast species. In a foreign species, nucleosome depletion at promoters is maintained over poly(dA:dT) tracts, whereas internucleosome spacing and all other aspects of nucleosome positioning tested are not. Interestingly, the locations of the +1 nucleosome and RNA start sites shift in concert. Strikingly, in a foreign species, nucleosome-depleted regions occur fortuitously in coding regions, and they often act as promoters that are associated with a positioned nucleosome array linked to the length of the transcription unit. We suggest a three-step model in which nucleosome remodelers, general transcription factors, and the transcriptional elongation machinery are primarily involved in generating the nucleosome positioning pattern in vivo.

The role of nucleosome positioning in the evolution of gene regulation

Chromatin organization plays a major role in gene regulation and can affect the function and evolution of new transcriptional programs. However, it can be difficult to decipher the basis of changes in chromatin organization and their functional effect on gene expression. Here, we present a large-scale comparative genomic analysis of the relationship between chromatin organization and gene expression, by measuring mRNA abundance and nucleosome positions genome-wide in 12 Hemiascomycota yeast species. We found substantial conservation of global and functional chromatin organization in all species, including prominent nucleosome-free regions (NFRs) at gene promoters, and distinct chromatin architecture in growth and stress genes. Chromatin organization has also substantially diverged in both global quantitative features, such as spacing between adjacent nucleosomes, and in functional groups of genes. Expression levels, intrinsic anti-nucleosomal sequences, and trans-acting chromatin modifiers all play important, complementary, and evolvable roles in determining NFRs. We identify five mechanisms that couple chromatin organization to evolution of gene regulation and have contributed to the evolution of respiro-fermentation and other key systems, including (1) compensatory evolution of alternative modifiers associated with conserved chromatin organization, (2) a gradual transition from constitutive to trans-regulated NFRs, (3) a loss of intrinsic anti-nucleosomal sequences accompanying changes in chromatin organization and gene expression, (4) re-positioning of motifs from NFRs to nucleosome-occluded regions, and (5) the expanded use of NFRs by paralogous activator-repressor pairs. Our study sheds light on the molecular basis of chromatin organization, and on the role of chromatin organization in the evolution of gene regulation.