Humanized nucleosomes reshape replication initiation and rDNA/nucleolar integrity in yeast

Eukaryotic DNA wraps around histone octamers forming nucleosomes, which modulate genome function by defining chromatin environments with distinct accessibility. These well-conserved properties allowed "humanization" of the nucleosome core particle (NCP) in Saccharomyces cerevisiae at high fitness costs. Here we studied nucleosome-humanized yeast-genomes to understand how species-specific chromatin affects nuclear organization and function. We found a size increase in human-NCP, linked to shorter free linker DNA, supporting decreased chromatin accessibility. 3-D humanized-genome maps showed increased chromatin compaction and defective centromere clustering, correlated with high chromosomal aneuploidy rate. Site-specific chromatin alterations were associated with lack of initiation of early origins of replication and dysregulation of the ribosomal (rDNA and rRNA) metabolism. This latter led to nucleolar fragmentation and rDNA-array instability, through a non-coding RNA dependent mechanism, leading to its extraordinary, but entirely reversible, intra-chromosomal expansion. Overall, our results reveal species-specific properties of the NCP that define epigenome function across vast evolutionary distances. HighlightsHumanized nucleosomes wrap 10 additional nucleotides, shortening free linker length Histone-humanized nucleosomes have increased occupancy for DNA Humanized nucleosomes potentially decrease chromatin accessibility by blocking-out free linker DNA Nucleosome humanization impedes DNA replication by affecting chromatin structure at origins Humanized nucleosomes reversibly destabilize the ribosomal DNA array and leads to massive intrachromosomal rDNA locus expansion Histone humanization disrupts rDNA silencing and leads to nucleolar fragmentation