Abstract
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.