The C domain of ARS121 compensates for the destabilizing effects of the mcm1-1 mutation on minichromosome maintenance. (A) Plasmid loss rates of YCp1, YCpAB121, and YCp121 containing ARS1(853 bp), ARS121 (489 bp), and ARS121(877 bp), respectively. ACS is symbolized by shaded arrowheads. Abf1 binding sites are symbolized by filled circles. Thick bar represents AT-rich sequences containing near matches to the ACS found in the B domain of ARS121. (B) Autoradiograms of replicating DNA prepared from isogenic wild-type (WT) and mcm1-1 mutant strains separated by 2-D gel electrophoresis. Filled arrows point to initiation bubbles or “bubble” arcs. Unfilled arrows point to elongation forks or “Y” arcs. Portions of the results shown in panels A and B are reproduced from reference 17.