THE MOD-A SUPPRESSOR OF NONALLELIC HETEROKARYON INCOMPATIBILITY IN PODOSPORA-ANSERINA ENCODES A PROLINE-RICH POLYPEPTIDE INVOLVED IN FEMALEORGAN FORMATION

Vegetative incompatibility in fungi results from the control of hetero karyon formation by the genes present at het loci. Coexpression of ant agonistic het genes in the same hyphae leads to a lethal process. In P odospora anserina, self-incompatible strains containing nonallelic inc ompatible genes in the same nucleus are inviable as the result of a gr owth arrest and a lytic process. Mutations in suppressor genes (mod ge nes) can restore the viability. These mod mutations also interfere wit h developmental processes, which suggests common steps between the inc ompatibility reaction and cellular differentiation. The mod-A locus, r esponsible for growth arrest in the self-incompatible strains, is also involved in the control of the development of female organs. The mod- A gene was isolated. An open reading frame 687 amino acids long was id entified. The MOD-A-encoded polypeptide is rich in proline residues, w hich are clustered in a domain containing a motif that displays simila rity to SH3-binding motifs, which are known to be involved in protein- protein interactions. Construction of a strain deleted for mod-A confi rmed that the product of this gene involved in differentiation is a ke y regulator of growth arrest associated with vegetative incompatibilit y.