Mutational blockage of DNA synthesis in Paramecium tetraurelia

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Abstract
One hundred and ninety-eight temperature-sensitive mutants of Paramecium tetraurelia were isolated after nitrosoguanidine mutagenesis. In some experiments, mutants were recovered with the aid of a bromouracil (BU) selection system. Fifty-six mutants showed cessation of cell division within one cell cycle at the restrictive temperature and were designated ts-0. Fourteen of the ts-0's showed a greater than 90% reduction in rnacronuclear deoxyribonucleic acid (DNA) synthesis at the restrictive temperature. Two ts-0. DNA-defective lines continued protein synthesis at greater than 50% the normal rate after arrest of DNA synthesis. Hence, these two mutants may be directly affected in the replication process itself. The two mutants are allelic and, in addition, a third 'leaky' allele was recovered. Comparison of experiments in which either BU selection or no selection was employed shows that a greater than 10-fold enrichment for ts mutants resulted from BU selection.