Figure 7.

 Mitotic arrest in cds1 mutants is dependent on Chk1 activation. (A) Synchronous cultures of appropriate strains were tested for the mitotic checkpoint in response to HU as in Fig. 1. (•) −Hu; (□) + HU. (B) The phosphorylation-dependent mobility shift of Chk1 kinase was assayed in cds1⁺ and cds1 null mutant cells containing an integrated triple HA-tagged chk1 gene (Walworth and Bernards 1996). Samples from asynchronous cultures treated with 12 mm HU were taken at the times indicated. Chk1 was detected, as described previously (Walworth and Bernards 1996), with anti-HA monoclonal. In cds1⁺ cells, Chk1 remains unphosphorylated, whereas in cds1 null mutant cells Chk1 is rapidly phosphorylated.