Dominant and semidominant mutations leading to thermosensitivity of ribonucleic acid biosynthesis in Saccharomyces cerevisiae.

Different dominant thermosensitive mutations affecting the same gene were selected in Saccharomyces cerevisiae. Ribonucleic acid (RNA) synthesis decreased rapidly and markedly at 37 C in all the mutants whether they were in a homozygous or a heterozygous state. Protein biosynthesis was at first unaffected and then decreased slowly, stopping after 5 h. Measurements of RNA biosynthesis in isolated nuclei as well as in vitro activities of RNA polymerases A and B at 22 and 37 C failed to reveal any difference between mutants and the wild type. Analysis of the nature of the residual RNAs synthesized at the high temperature in the mutants showed a small relative increase in the messenger RNA fraction, but it was not sufficient to indicate a specific inactivation of RNA polymerase A activity. The results suggest an impairment in a common regulatory element for all RNA polymerases acting at the level of the initiation of transcription. Similar mutants with a semidominant phenotype were obtained in which the lesions were in two other unlinked loci.