Deletion of med18+or med20+impairs centromere function. (A) A non-essential mini-chromosome, Ch16m23::ura4+-Tel, is frequently lost in strains deleted for med18+, med20+ or clr4+. Cells containing the mini-chromosome form white colonies on medium with low concentration of adenine while cells lacking the mini-chromosome form red colonies. Loss of the mini-chromosome in the first cell division after plating results in a half-sectored colony. (B) Deletion of med18+ or med20+ renders the cells sensitive to the microtubule destabilizing agent thiobendazole (12 μg/ml). The strains for this figure were: WT (FY520), med18 Δ (TP527), med20 Δ (TP527), and clr4Δ (PG3420).