Second-site, intragenic alterations in the gene encoding subunit II of cytochrome c oxidase from yeast can suppress two different missense mutations

Abstract.

Cytochrome c oxidase, a multi-subunit enzyme complex, accepts electrons from cytochrome c and transfers them to molecular oxygen to form water. Subunit II (Cox2p) of the enzyme complex provides the initial entry site for the electrons from cytochrome c. We report here the characterization of a yeast strain bearing a mutation in the gene encoding Cox2p which abolishes the activity of the enzyme complex. The alteration, at residue 163 in the yeast polypeptide, substitutes isoleucine for threonine and leads to loss of Cox2p and loss of the ability to carry out cellular respiration. We have also characterized 55 independent revertants of the mutant which have recovered the ability to respire. Of these revertants, 37 recover the ability to respire due to a compensatory alteration at residue 163, which produces either a wild-type threonine codon or one for valine or serine. The other 18 revertants recover function due to secondary changes at four different codons within the gene encoding Cox2p. Some of these second-site, intragenic revertants occur at sites significantly distant from the position of the original mutation. In addition, alterations at two of these sites have previously been shown to suppress a completely different missense mutation in the gene.