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Abstract

198 temperature-sensitive mutants of Paramecium aurelia were isolated following nitrosoguanidine mutagenesis. In some experiments, mutants were recovered with the aid of a bromouracil (BU) selection system developed by Baumann (1973). 56 mutants showed cessation of cell division within one cell cycle at the restrictive temperature and were designated ts-0.14 of the ts-0's showed a greater than 90% reduction in macronuclear DNA synthesis at the restrictive temperature. Two ts-0, DNA-defective lines continued protein synthesis at greater than 50% the normal rate following 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 ten-fold enrichment for ts mutants resulted form BU selection.