CONTROL OF MATING AND DEVELOPMENT IN USTILAGO-MAYDIS

In the fungus Ustilago maydis, the ability to distinguish between part ners that are of the same or of different mating type is controlled by two mating-type loci. One locus allows extracellular recognition thro ugh a pheromone-based system. After cell fusion, the other mating-type locus, which exists in multiple alleles, determines intracellular rec ognition. Each allele encodes a pair of homeodomain proteins that are active only in pairwise combinations in which the two partners origina te from different alleles of the locus. Recent discoveries suggest tha t the underlying molecular recognition mechanism is the ability to for m heterodimers. Whereas the proteins in all different allelic combinat ions interact, it is a specific feature of proteins from the same alle le not to interact. This suggests the existence of a code for protein- protein recognition.