The importance of mutation, then and now: studies with yeast cytochrome c

The development of a genetic system based on the CYC1 gene was initiated over 40 years ago, primarily because of the anticipated ease of sequencing of the corresponding encoded protein, iso-1-cytochrome c from Saccharomyces cerevisiae. The success of the iso-cytochrome c system was dependent on the early development of methods for detecting and selecting cyc1 defective mutants and CYC1 functional revertants, and of methods for fine-structure genetic mapping using deletions and single-site mutations. The nonsense codons TAA and TAG, and the initiation codon ATG, were determined from the amino acid alterations of iso-1-cytochromes c from intragenic revertants; this represented the first assignments of such codons in a eukaryotic organism. The types of desired sequences were expanded by selecting recombinants from cyc1 × cyc1 nonfunctional mutants or CYC1 × CYC1 functional mutants, permitting the early determination of the rules of translation, which differed from those of prokaryotes by use of the most 5′ AUG codon for initiation of translation. The sequence of 44 base pairs of CYC1 was determined with altered iso-1-cytochromes c from revertants of frameshift and initiation mutants, allowing the early cloning of the gene. A method was developed for transforming yeast directly with synthetic oligonucleotides, resulting in the convenient production of CYC1 mutants with defined sequences. At this point in time, Sherman and colleagues have published approximately 240 papers on or using the iso-cytochrome c system, dealing with such diverse topics as translation, informational suppressors, transcription and transcription termination, recombination, ectopic recombination, mutagen specificity, regulation by Ty1 elements, evolution of duplicated chromosomal segments, structure-function relationships of cytochrome c, protein stability and degradation, biosynthesis and mitochondrial import of cytochrome c, mitochondrial proteases, co- and post-translational modifications, and mRNA degradation. Current work on degradation of proteins in mitochondria, on degradation of mRNA in the nucleus, and on N-terminal acetylation stems from properties of CYC1 mutants isolated in early screens more than a decade ago.