Breaking Symmetry − Asymmetric Histone Inheritance in Stem Cells Jing Xie, Matthew Wooten, Vuong Tran, Xin Chen  Trends in Cell Biology  Volume 27, Issue 7, Pages 527-540 (July 2017) DOI: 10.1016/j.tcb.2017.02.001 Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 1 A Two-Step Model to Explain the Asymmetric H3 Inheritance in Drosophila Male Germline Stem Cells (GSCs) Asymmetric Cell Division (ACD). Step one: the old (green) and the new (red) H3 histones are differentially deposited on the two chromatids at the gene locus. Step two: the sister chromatids with asymmetric H3 are asymmetrically segregated. The mother centrosome is in proximity to the stem cell niche. Trends in Cell Biology 2017 27, 527-540DOI: (10.1016/j.tcb.2017.02.001) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 2 Proposed Mechanism for Directed Histone Recycling during DNA Replication. DNA replication overview: polymerase ε synthesizes the leading strand continuously, whereas polymerase δ synthesizes the lagging strand in discontinuous segments known as Okazaki fragments. Polymerase α synthesizes an RNA primer at the start of the leading strand (not shown) and at the start of each new Okazaki fragment. Old histone recycling: the MCM2-7 helicase sits at the foremost edge of the replication fork where its primary function is to unwind non-replicated DNA. The MCM2 protein can bind to (H3–H4)2 tetramers dissociated in the wake of the advancing fork to allow their subsequent deposition on nascent DNA. Owing to its ability to bind (H3–H4)2 tetramers, CAF-1 may help to coordinate the deposition of pre-existing histones after fork passage. CAF-1 is recruited to the edge of the advancing fork by PCNA, which also serves as a processivity factor for the replicative polymerases δ and ε. New histone deposition: histone chaperone ASF-1 binds to (H3–H4) dimers in the cytoplasm and coordinates their import into the nucleus where ASF-1 transfers (H3–H4) dimers to CAF-1. CAF-1 then coordinates new H3–H4 deposition onto nascent DNA. The subsequent incorporation of two (H2A–H2B) dimers reforms the nucleosome structure and signifies the end of the process of replication-coupled histone deposition. Trends in Cell Biology 2017 27, 527-540DOI: (10.1016/j.tcb.2017.02.001) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 3 Proposed Models for Directed Histone Recycling at the Replication Fork. (A) Semi-conservative model: old (H3–H4)2 tetramers are split at the replication fork, possibly by ASF1, with the resulting (H3–H4) dimers being inherited equally on both the leading and the lagging strands. (B) Dispersive model: old (H3–H4)2 tetramers remain together at the fork and are randomly partitioned between the leading and lagging strands to ensure global symmetry of old versus new on newly synthesized daughter strands. (C) Conservation model: old (H3–H4)2 tetramers remain unsplit at the fork and are inherited as a tetramer. In this model, tetramers are biased in their inheritance such that either the leading strand (shown) or the lagging strand (not shown) inherits the majority of the old (H3–H4)2 tetramers. Trends in Cell Biology 2017 27, 527-540DOI: (10.1016/j.tcb.2017.02.001) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 4 Sequential Phosphorylation at Thr3 of Histone H3 (old) before H3 (new) Ensures Asymmetric Sister Chromatid Segregation. Abbreviations: H3T3P, histone H3 phosphorylated at Thr3; WTH3, wild-type H3. Trends in Cell Biology 2017 27, 527-540DOI: (10.1016/j.tcb.2017.02.001) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 5 Both Dominant Negative H3T3A and Phosphomimetic H3T3D Randomize the Segregation Pattern of Sister Chromatids. Abbreviations: H3T3A, Thr3 of histone H3 mutated to the unphosphorylatable residue alanine; H3T3D, Thr3 mutated to the phosphomimetic aspartic acid. Trends in Cell Biology 2017 27, 527-540DOI: (10.1016/j.tcb.2017.02.001) Copyright © 2017 Elsevier Ltd Terms and Conditions