A further two mutants defective in initiation of the S phase in the yeastSaccharomyces cerevisiae
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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.
KeywordsCell Cycle Protein Synthesis Restrictive Condition Restrictive Temperature Final Requirement
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