Abstract

Histone Inheritance Chromatin, the packaging material for eukaryotic genomes, is a potential repository for epigenetic information. The core structure of chromatin is the nucleosome, which consists of an octamer of histone proteins, two dimers each of histones H2A and H2B, and histones 3 and 4. Histones 3 and 4, in particular, carry a series of covalent modifications presumed to be passed on through cell division. Using mass spectrometry of tagged and isotope labeled histones, Xu et al. (p. 94 ; see the Perspective by Ray-Gallet and Almouzni ) followed the inheritance of the histones themselves through mitosis. The H2A-H2B dimers were inherited randomly through cell division, correlating with their lack of major covalent marks. In comparison, replication-deposited H3.1-H4 dimers did not separate through cell division, implying that H3 and H4 histone modifications might be maintained by copying from neighboring preexisting histones. Intriguingly, up to one-quarter of the nonreplication-deposited H3.3-H4 dimers, which mark active chromatin, did split during cell division.