A further two mutants defective in initiation of the S phase in the yeastSaccharomyces cerevisiae

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Abstract
Two new mutantsdbf3 and4, are specifically defective in DNA synthesis. When synchronous cultures ofdbf4 were transferred from the permissive to restrictive temperature before the start of the S phase, no DNA synthesis occurred. However when switched after the beginning of DNA replication, the cells completed that round of synthesis.Dbf4 therefore resemblescdc7, a mutant believed to be required for initiation of DNA synthesis, and indeeddbf4 acts at a point in the cell cycle close tocdc7, namely betweencdc4 and the final requirement for protein synthesis before the S phase. Likedbf4, cultures ofdbf3 transferred to restrictive conditions before the start of S showed little DNA synthesis. However a small burst did occur at a time roughly corresponding to when normal initiation would be expected. Cultures switched after this time completed the ongoing round of DNA synthesis.