Significance of ribosomal ribonucleic acid synthesis for control of the G1 period in the cell cycle of the heterobasidiomycetous yeast Rhodosporidium toruloides.

A cell cycle mutant strain which is defective in the G1 period, B2-39, was selected from 1,200 temperature-sensitive mutants of the heterobasidiomycetous yeast Rhodosporidium toruloides M-1057. In the mutant cells, ribosomal ribonucleic acid synthesis was initially inhibited upon temperature shift-up from a permissive (25 degrees C) to a restrictive (36 degrees C) temperature. Moreover, the mutant was found to be temperature sensitive in deoxyribonucleic acid-dependent ribonucleic acid polymerase I activity in vitro. In a revertant-mutant strain, B2-39-R-2, both ribosomal ribonucleic acid synthesis in vivo and enzyme activity in vitro were simultaneously recovered. These results indicate that the mutant has a temperature-sensitive, deoxyribonucleic acid-dependent ribonucleic acid polymerase I and suggest that ribosomal ribonucleic acid synthesis acts as one of the control factors for initiation of both deoxyribonucleic acid synthesis and bud emergence.