The telomerator drives in vivo linearization of a synthetic yeast chromosome arm, synIXR BAC. (A) synIXR BAC is a ∼100-kb circular chromosome arm that can power a yeast cell. In independent integration experiments, the telomerator cassette was introduced by homologous recombination at two gene-free regions of synIXR BAC as indicated (integration site 1, ∼40 kb; integration site 2, ∼80 kb). (B) Following 24 h of growth in galactose-containing medium to induce expression of I-SceI, single cells from integration site 1 (Left) and integration site 2 (Right) were plated on YPD medium, and then replica plated onto SC medium supplemented with 5′-fluoroorotic acid (Foa), and SC medium lacking uracil or leucine (SC–Ura and SC–Leu, respectively). Only in cell populations encoding an intact I-SceI recognition sequence as part of the telomerator cassette (+ I-SceI; Top) did linearization occur, as assayed by appearance of colonies displaying growth on Foa and nongrowth on SC–Ura.