Abstract
Nucleosomes are the fundamental structural unit of chromatin. In addition to stabilizing the DNA polymer, nucleosomes are modified in ways that reflect and affect gene expression in their vicinity. It has long been assumed that nucleosomes can transmit memory of gene expression through their covalent posttranslational modifications. An unproven assumption of this model, which is essential to most models of epigenetic inheritance, is that a nucleosome present at a locus reoccupies the same locus after DNA replication. We tested this assumption by nucleating a synthetic chromatin domain in vivo, in which ∼4 nucleosomes at an arbitrary locus were covalently labeled with biotin. We tracked the fate of labeled nucleosomes through DNA replication, and established that nucleosomes present at a locus remembered their position during DNA replication. The replication-associated histone chaperones Dpb3 and Mcm2 were essential for nucleosome position memory, and in the absence of both Dpb3 and Mcm2 histone chaperone activity, nucleosomes did not remember their position. Using the same approach, we tested the model that transcription results in retrograde transposition of nucleosomes along a transcription unit. We found no evidence of retrograde transposition. Our results suggest that nucleosomes have the capacity to transmit epigenetic memory across mitotic generations with exquisite spatial fidelity.