Genetic Evidence for the Creation of a Reinitiation Site by Mutation Inside the Yeast Ura 2 Gene

The ura 2 gene of yeast codes for two enzymatic activities which are translated from a unique messenger RNA in the order carbamoyl-phosphate synthetase (CPSase), aspartate transcarbamylase (ATCase) (Lacroute, 1968; Denis-Duphil and Kaplan, 1976). Nonsense mutations in the CPSPase region cause a complete loss in ATCase activity by a total polar effect, characteristic of eukaryotic mRNA translation, and due to the unique site of protein initiation present on each messenger (Shaffer et al., 1969). A triple nonsense mutant in the CPSase has been constructed by recombination and ATCase+ revertants have been selected from it. Among seventeen revertants obtained, three had a deletion covering the three nonsense mutations relieving thus the polar effect (Fink and Styles, 1974) but fourteen others examined had retained all the CPSase DNA including the three nonsense mutations; this can be explained in the present state of knowledge only by the creation by mutation of reinitiation site either for transcription or for translation in the region of the ura 2 gene distal to the last nonsense mutation.