A further two mutants defective in initiation of the S phase in the yeast Saccharomyces cerevisiae.

Two new mutants dbf3 and 4, are specifically defective in DNA synthesis. When synchronous cultures of dbf4 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 resembles cdc7, a mutant believed to be required for initiation of DNA synthesis, and indeed dbf4 acts at a point in the cell cycle close to cdd7, namely between cdd4 and the final requirement for protein synthesis before the S phase. Like dbf4, cultures of dbf3 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.