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. 2020 Apr 20;215(2):393–406. doi: 10.1534/genetics.120.303264

Figure 1.

Figure 1

Model of sister chromatid transposition-induced DNA rereplication. (A) Maize chromosome 1 and progenitor allele p1-vv-9D9A with DNA replication bubbles. Solid circle indicates centromere. The p1 gene contains 3 exons (black boxes) with Ac and fAc elements located in intron 2 (gray boxes with solid/open arrowheads indicate the 5′ and 3′ Ac/fAc termini, respectively). The 5′ and 3′ Ac/fAc termini involved in sister chromatid transposition are circled in red and blue, respectively. The unreplicated regions are highlighted in yellow, and the replicated regions to be rereplicated are highlighted in blue. (B and C) Sister chromatid transposition, p1-ww-id1 orientation. (B) Excision of the Ac 5′ and fAc 3′ termini results in excision footprint (marked by X) and fusion of the two sister chromatids. Curved arrows indicate insertion of the excised transposon ends into the target site (short vertical line). (C) The 5′ terminus of Ac is ligated to the centromere-proximal side (marked α) and the 3′ terminus of fAc is ligated to the distal side (marked β). This joins the unreplicated sequences at the insertion site to the previously replicated Ac/fAc sequences. The replication fork containing the sister chromatid fusion is flipped. (D) DNA replication. As DNA replication continues, Ac/fAc sequences are rereplicated (highlighted in green). (E) Completion of DNA replication. Rereplication of Ac/fAc sequences aborts, releasing two broken ends that fuse (dotted line). This produces a Composite Insertion between α and β. During mitosis, sister chromatids separate. One sister chromatid (top) has a deletion, and the other sister chromatid (bottom) carries a corresponding inverted duplication and a Composite Insertion between α and β. (F) Maize chromosome 1 with the p1-ww-id1 allele. The Composite Insertion in the α/β site contains two new fAc elements (solid red arrows) containing Ac 5′ sequences in inverted orientation: fAc(P) (proximal) and fAc(D) (distal). P1-ww-id1 also contains an inverted duplication of the segment from fAc(O) (original) to site β. (G and H) Sister chromatid transposition, p1-ww-id4 orientation. (G) Same as B, except the 3′ terminus of fAc will ligate to the centromere-proximal side (marked γ) and the 5′ terminus of Ac will ligate with the distal side (labeled as δ) of the insertion site. (H–J) Same as C–E. (K) Maize chromosome 1 containing the p1-ww-id4 allele. For animation of alternative transposition mechanism, see Supplementary Materials Video 1.