Intragenic revertants of yeast invertase variants with secretion-defective leader sequences

Several secretion-defective variants of invertase from Saccharomyces cerevisiae were generated by replacement of the wild-type signal sequence codons with DNA fragments with random sequences. Strains encoding these proteins failed to grow on medium containing sucrose as the sole source of carbon. The invertase that was made in these strains was found to fractionate with soluble, cytoplasmic proteins, and indirect immunofluorescence confirmed that the mutant invertase was located throughout the cytoplasm. To define the defects in the secretion-defective leader sequences, we selected revertants by requiring growth on sucrose. Surprisingly, most of the reversion events consisted of point changes and duplications in the upstream noncoding portion of the gene. Each of these changes introduced several hydrophobic residues into the nonfunctional leader sequences, suggesting that the defective random leader peptides might simply lack adequate hydrophobicity to be effective signal peptides.