Page 43 - Annual Report 2020

transmitted to daughter cells, but will also provide new opportunities to understand human cancer caused by the alteration in epigenetic states.
Previous studies on nucleosome metabolism mainly focus on the assembly and deposition of newly synthesized histones, including (H3-H4)2 tetramers. How parental histone (H3-H4)2 tetra- mers are transferred and assembled into nucle- osomes behind DNA replication forks remains unclear. The current dominant model proposes that parental (H3-H4)2 tetramers are randomly distributed onto leading and lagging strands of DNA replication forks based on the experiments of pharmacological inhibition of the new protein synthesis. However, this model has never been tested in a non-stress condition, and histone deposition at replication forks originating from different loci may be different. Moreover, the protein(s) responsible for transferring parental (H3-H4)2 tetramers have not been identified.
We find that parental histones are distributed to both the leading and lagging strand arm of the fork with a slight bias towards the lagging strand. Intriguingly, in cells depleted of Dpb3 and Dpb4, which are non-catalytic subunits of the leading strand DNA polymerase, Pol-epsilon, the bias of
parental nucleosome segregation to the lagging strand was markedly enhanced. Polepsilon thus appears to mediate the recruitment of parental nucleosomes to the leading strand.
Mutation of the histone H3-H4 binding domain
in the Mcm2 subunit of the replicative helicase causes a strong bias of parental nucleosome partitioning to the leading strand, indicating that Mcm2 targets parental nucleosomes for depo- sition on the lagging strand. that parental H3-H4 tetramers bind to Mcm2, which travels along the leading-strand template, and are then transferred to the lagging strand by the Mcm2-Ctf4-Pola complex for nucleosome assembly.
While cellular DNA replication was not grossly affected during the mitotic cell cycle by asym- metric nucleosome partitioning, we found that silenced mating type locus was significantly disturbed in the Dpb3/ Dpb4 and mcm2 mutant budding yeast cells. Together our studies thus
suggest that parental nucleosome segregation
is determined by a balance of replisomeintrinsic histone chaperone activities. Modulation of indi- vidual histone chaperone activities at the fork may therefore provide a potential mechanism for the asymmetric distribution of epigenetic marks to determine cell fate in development.
Other professional activities:
Publications
Yang, Y. & Yu, C. Liquid phase condensation directs nucleosome epigenetic modifica- tions. Signal Transduct Target Ther 5, 64 (2020).
Current research projects:
1) Candidategenescreeningon parental histone assembly in yeast
2) Functionalanalysisonepigenetic inheritance of DNA polymerase and MCM helicase
3) NewapproachestostudyDNA replication coupled process
Figure 1: A model for parental histone (H3-H4)2 tetramer transfer to replicating DNA strands. (1) the parental histone (H3-H4)2 tetramer transfer is not random but biased toward the replication lagging strand; and that (2) DNA polymerase ε and MCM helicase mediate the parental histone transfer to the replication leading and lagging strands, respectively.
THE HORMEL INSTITUTE // UNIVERSITY OF MINNESOTA PG 43