Nuclear and Mitochondrial Informational Suppressors of box3 Intron Mutations in Saccharomyces cerevisiae

Abstract

The recent DNA sequencing of the wild-type and mutated box3 intron of the mosaic mitochondrial cob-box gene has led to postulation of an introncoded mRNA-maturation protein responsible for splicing (Jacq et al. 1980; Lazowska et al. 1980). Thus, studies of suppressors of box3 mutations seem of particular interest. This paper describes genetic studies of suppressors of box3 intron mutations using essentially the methodology developed by Dujardin et al. (1980a).

RESULTS
A total of 953 revertants from 17 different box3 mutants were isolated, and their genetic determinants and phenotypes were characterized. Most revertants were of spontaneous origin; their characteristics are given in Table 1. The great majority of revertants were mitochondrially determined, and most of them had a wild-type phenotype. Nuclear suppressors were found in some mutants only. All of these suppressors were recessive and had a pseudo-wild phenotype. Three of them, nam3-1, nam4-1, and nam5-1, isolated from box3 mutant M2101, were chosen for further studies. They showed 2:2 segregation in meiosis and a lack of mitotic segregation.

Mitochondrial suppressors unlinked with the box3 target mutation were searched for among revertants of pseudo-wild phenotype. Of 26 revertants tested (isolated from seven different box3 mutants), one such suppressor (mim3-1), derived from a box3-9 mutant, was found. It was detected because, in a cross with the wild-type strain (box3⁻sup^act × box3⁺sup^ina), it gave a significantly higher frequency of Gly⁻ colonies (16%) than did the other revertants (0.9–3.3%) and the control cross of the two wild-type strains (1.5%). Gly⁻ colonies from the cross...