THE MOLECULAR NATURE OF MUTATIONS IN THE MT A-1 GENE OF THE NEUROSPORA-CRASSA-A IDIOMORPH AND THEIR RELATION TO MATING-TYPE FUNCTION

The 293-amino acid mt A-1 ORF of the A mating-type idiomorph of Neuros pora crassa is multifunctional. It confers A mating identity and is re sponsible for heterokaryon incompatibility. The goal of this study was to dissect the functional regions of mt A-1. New mutants of mt A-1 se lected for loss of the incompatibility function were obtained. One new mutant, A(m99), was partially fertile as a maternal parent. This is t he first time that fertility and incompatibility functions have been s eparated for the A idiomorph. In this mutant, the mt A-1 ORF is trunca ted after the first 85 amino acids, indicating that this N-terminal re gion is minimally sufficient for female fertility. A series of deletio n constructs and frameshift alleles of mt A-1 was obtained and tested for male-mating activity and vegetative incompatibility in transformat ion experiments. These experiments showed that a region from position 1 to 111 is sufficient to confer incompatibility, while amino acids fr om position 1 to 227 are required for mating activity. A transcription al analysis of mt A-1 showed that the mRNA is expressed both before an d after fertilization. This, together with the phenotype of the A(m99) mutant, suggests a post-fertilization function for mt A-1.