Fig. 1. p100-Ciz1 is co-regulated with cyclin A-cdk2-induced DNA replication. (A) Anti-Cdc6 antibody V detects mouse Cdc6 and a second antigen in western blots of 3T3 whole cell extract, which migrates with an approximate molecular mass of 100 kDa. p100 is present in both the soluble fraction and insoluble nuclear fraction (prepared under in vitro replication conditions). (B) Initiation of DNA synthesis in `replication competent' late G1 phase nuclei by G1 extract supplemented with recombinant cyclin A-cdk2. Control bar shows the proportion of nuclei already in S phase (unshaded), and those that initiated replication in extract from S phase cells (shaded). (C) Late G1 nuclei re-isolated after incubation in parallel reactions with recombinant cyclin A-cdk2 and G1 extract, probed for Mcm2 and Mcm3. (D) p100 antigen is more abundant in nuclei exposed to initiation-inducing concentrations of cyclin A-cdk2, revealed when the same nuclei are probed with antibody V.