RT Journal Article SR Electronic T1 Asymmetric histone incorporation during DNA replication in Drosophila male germline stem cells JF bioRxiv FD Cold Spring Harbor Laboratory SP 242768 DO 10.1101/242768 A1 Matthew Wooten A1 Jonathan Snedeker A1 Zehra Nizami A1 Xinxing Yang A1 Elizabeth Urban A1 Ashlee Feng A1 Jee Min Kim A1 Joseph Gall A1 Jie Xiao A1 Xin Chen YR 2018 UL http://biorxiv.org/content/early/2018/08/23/242768.abstract AB Many stem cells undergo asymmetric division to produce both a self-renewing stem cell and a differentiating daughter cell. We previously reported that in male Drosophila germline stem cells (GSCs), preexisting (old) histone H3 is inherited to the self-renewed stem cell, whereas the differentiating daughter is enriched with newly synthesized (new) H3. However, the molecular mechanisms that establish asymmetric H3 distribution on sister chromatids were unclear. Here, we show that histone H4 is likewise inherited asymmetrically, while histones H2A and H2B are both inherited symmetrically. We hypothesize that the asymmetric distribution of H3 and H4 at sister chromatids is established during DNA replication. To directly visualize histone incorporation pattern on replicating sister chromatids, we develop a chromatin fiber technology with differential labeling of old versus new histones. We find spatially separable distributions of old and new H3 on isolated replicating sister chromatids. By contrast, old and new H2A displayed symmetric distribution on replicating sister chromatids. Furthermore, co-localization studies on chromatin fibers and proximity ligation assays on intact nuclei reveal that old H3 are preferentially retained by the leading strand while new H3 preferentially associate with the lagging strand. Finally, using a sequential nucleoside analog incorporation assay, we detect significantly increased unidirectional DNA replication on chromatin fibers from germline compared to somatic cells. Together, the unidirectionality of fork movement coupled with the strandness of histone incorporation could explain the asymmetry between old and new H3 on replicated sister chromatids. In summary, these results indicate that the spatial and temporal asymmetries inherent to DNA replication process may serve to bias histone incorporation, suggesting an unappreciated role for DNA replication in asymmetrically dividing cells.