Among 150 temperature-sensitive Saccharomyces cerevisiae mutants which we have isolated, 15 are specifically affected in ribonucleic acid (RNA) synthesis. Four of these mutants exhibit particularly drastic changes and were chosen for a more detailed study. In these four mutants, RNA synthesis is immediately blocked after a shift at the nonpermissive temperature (37 C), protein synthesis decays at a rate compatible with messenger RNA half-life, and deoxyribonucleic acid synthesis increases by about 40%. All the mutations display a recessive phenotype. The segregation of the four allelic pairs ts-/ts+ in diploids is mendelian, and the four mutants belong to three complementation groups. The elution patterns (diethylaminoethyl-Sephadex) of the three RNA polymerases of the mutants grown at 37 C for 3.5 h show very low residual activities. The in vitro thermodenaturation confirms the in vivo results; the half-lives of the mutant activities at 45 C are 10 times smaller than those of the wild-type enzymes. Polyacrylamide gel electrophoresis shows that the synthesis of all species of RNA is thermosensitive. The existence of three distinct genes, which are each indispensable for the activity of the three RNA polymerases in vivo as well as in vitro, strongly favors the hypothesis of three common subunits in the three RNA polymerases.

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Thonart P, Bechet J, Hilger F, Burny A

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