HOMOLOGOUS RECOMBINATION PARTLY RESTORES THE SECRETION DEFECT OF UNDERGLYCOSYLATED ACID-PHOSPHATASE IN YEAST

The majority of secreted acid phosphatase in Saccharomyces cerevisiae is encoded by the PHO5 gene, The secretion level of this acid phosphat ase is directly determined by its level of glycosylation. Consequently , PHO5-11-encoded acid phosphatase which lacks 11 of 12 glycosylation sites is only poorly secreted. We have isolated and characterized both UV-and EMS-induced variants, which are partly able to restore the sec retion of acid phosphatase. Our data indicate that the improved secret ion is caused by mitotic intrachromosomal recombination between the PH O5-11 allele and the homologous tandemly repeated PHO3 sequences, resu lting in the restoration of glycosylation sites in PNO5-11. Two differ ent recombination mechanisms, unequal sister-chromatid exchange and si ster-chromatid gene conversion, are responsible for these alterations of the PHO5-11 locus. Thus, recombination between mutant and wild-type sequences are able to restore the ability of mutant yeast cells to se crete acid phosphatase.