Sum1 and Hst1 were necessary for ARS activity of selected origins. A- Plasmid loss rates were measured in a wild-type (wt, w303), a sum1Δ (AEY3358) and an hst1Δ (AEY1499) strain. Strains carried CEN4-URA3 plasmids with ARS433 (pAE1240), ARS446 (pAE1250), ARS607 (pAE1242), ARS1109 (pAE1243), ARS1223 (pAE1130) or ARS1511 (pAE1135) as their sole origin. The loss rates are the average of three independent determinations. No loss rate could be determined for ARS446 in the sum1Δ strain. B- Primary transformants of wt, sum1Δ and hst1Δ strains carrying CEN4-URA3 plasmids with ARS446 (pAE1250), ARS606 (pAE1126) or AR1013 (pAE1081) were streaked on minimal plates lacking uracil and incubated for three days at 30°C. * (P < 0.05) and ** (P < 0.01) indicate statistically significant changes (student's t test).