cdc45-10 and dpb11-10 mutants exhibit a synthetic lethal interaction. (A) The shaded boxes in Dpb11p indicate repeated BRCT motifs. The Arg-to-Cys substitution encoded by dbp11-10 lies within a stretch of residues conserved in the S. pombe DPB11 homologue, Cut5. Identical residues are white text on black, and conserved residues are shaded gray. The cdc45-10 mutant encodes a Gly-to-Asp substitution. The shaded areas mark a putative bipartite nuclear localization signal. (B) Wild-type (WT), dpb11-10, cdc45-10, and three independently isolated double dpb11-10, cdc45-10 mutant strains were streaked onto YPD plates and were incubated at the indicated temperatures. All strains were top1-null.