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Proposed sequence of events in the evolution of Orc1/Sir3. In the ancestral state (Top) Orc1 acted with Sir2 to generate extended silenced domains. The first transition (Middle) was the acquisition of a Sir1-like protein and the emergence of a role for Orc1 as a silencer-binding protein. Subsequently (Bottom), a whole-genome duplication enabled the partitioning of functions between Orc1 and Sir3.
