S-phase, G2, and nuclear division mutants of Aspergillus nidulans.

Twenty-two temperature-sensitive cell cycle mutants of the fungus Aspergillus nidulans, which block in interphase at restrictive temperature, were analyzed by the reciprocal shift method of Jarvik and Botstein (Proc. Nath Acad. Sci. U.S.A. 70:2046-2050, 1973) and Hereford and Hartwell (J. Mol. Biol. 84:445-461, 1974) to determine whether these mutations were blocked at the G1, S, or G2 phase of the cell cycle. We found five mutants to be blocked in S and nine to be blocked in G2. Two of the G2 mutants were atypical in that they were not able to accomplish the G2 to M transition at restrictive temperature but nevertheless could initiate subsequent cycles of DNA replication. None was blocked in G1. There were nine strains that could not be classified. The block imposed by restrictive temperature was irreversible in three of these strains, and the six other strains were unclassifiable due to their aberrant terminal nuclear phenotypes.