Deletions and replacements of omega loops in yeast iso‐1‐cytochrome c

Ω(Omega)‐loops are protein secondary structural elements having small distance between segment termini. It should be possible to delete or replace certain of these Ω‐loops without greatly distorting the overall structure of the remaining portion of the molecule. Functional requirements of regions of iso‐1‐cytochrome c from the yeast Saccharomyces cerevisiae were in investigated by determining the biosynthesis and activity in vivo of mutant forms in which four different Ω‐loops were individually deleted, or in which one Ω‐loop was replaced with five different segments. Deletion encompassing amino acid positions 27–33 and79–83 either prevented synthesis of the holoprotein, or produced highly labile iso‐1‐cytochromes c , whereas deletions encompassing position 42–45 and 48–55 allowed partial synthesis and activity. These two latter regions, therefore, are not absolutely required for any biosynthetic process such as heme attachment, mitochondrial import, or for enzymatic interactions. All replacements in Loop A (residue position 24–33) with the same size (10 amino acid residues), longer (13 and 15 amino acid residues), or shorter segments (6 amino acid residues), resulted in strains having at least partial levels of iso‐1‐cytochrome c ; however, the relative activities ranged from zero to almost the normal level. Thus, Loop A does not appear to be essential for such biosynthetic steps as heme attachment and mitochondrial import. In contrast, the full range of relative activities suggest that this region interacts with physiological partners to carry out efficient electron transport.