We have identified the sua5 locus as a suppressor of an aberrant ATG codon located in the leader region of the cyc1 gene. The sua5-1 allele enhances the iso-1-cytochrome c steady state level in the cyc1-1019 mutant from 2% to approximately 60% of normal (Cyc+) and also confers a marked slow growth (Slg-) phenotype. Suppression is not a consequence of altered transcription initiation at the cyc1 locus. The SUA5 wild-type gene was isolated and sequenced, revealing an open reading frame (ORF) encoding a potential protein of 46,537 Da. SUA5 transcript analyses were consistent with expression of the predicted ORF and Sua5 antisera detected a protein with an apparent molecular mass of 44 kDa. SUA5 was mapped to chromosome VII, immediately adjacent to the PMR1 gene. Hybridization analysis revealed the presence of a related gene on chromosome XII. Neither the SUA5 DNA sequence nor deduced amino acid sequence showed homology to any sequences in the data banks. Disruption of SUA5 conferred the same Cyc+ and Slg- phenotypes as the sua5-1 suppressor, which is the result of a missense mutation, encoding a Ser107----Phe replacement. In addition, sua5 null mutants lack cytochrome a.a3 and fail to grow on lactate or glycerol medium. These results define SUA5 as a new gene encoding a novel protein that is necessary for normal cell growth.