Mutation of the gene for the second-largest subunit of RNA polymerase I prolongs the period length of the circadian conidiation rhythm in Neurospora crassa

The period length of the circadian conidiation rhythm was examined in a mutant strain of Neurospora crassa, un-18, that is temperature sensitive for mycelial growth. The un-18 mutant showed a temperature-sensitive phenotype with respect to both mycelial growth and the period length of the conidiation rhythm. Below 22°?C, the un-18 mutation did not affect the period length, but at temperatures between 22°?C and 32°?C, the period length of the un-18 mutant was ～2 h longer than that of the wild-type strain. The un-18 ⁺ gene was cloned and was found to encode the second-largest subunit of RNA polymerase I, which is involved in the synthesis of rRNA. These results indicate that a defect in ribosome synthesis, which must result in a lower rate of protein synthesis, lengthens the period of the circadian conidiation rhythm in Neurospora.