The mating-type alleles A and a of Neurospora crassa control mating in the sexual cycle and function in establishing heterokaryon incompatibility in the vegetative cycle. The A and a alleles were cloned, and they were shown to encode both the sexual functions and vegetative incompatibility. The mating-type clones contain nonhomologous DNA segments that are flanked by common DNA sequences. Neurospora crassa and all heterothallic and pseudohomothallic Neurospora species contain a single copy of one mating-type sequence or the other within each haploid genome. The six known self-fertile homothallic isolates contain an A homolog, but only one species also contains a homologous sequences. Homothallism in these species is not due to mating-type switching, as it is in Saccharomyces cerevisiae.

Translocation of neutral aliphatic and aromatic amino acids across the plasma membrane of the ascomycete Neurospora crassa requires a functional gene product of the mtr locus. Mutations at this locus are defective in transport of those amino acids. We have cloned the mtr+ gene of Neurospora crassa from an ordered cosmid library of genomic DNA and produced a preliminary restriction map of 2.9 kilobases of genomic DNA that encompasses the mtr coding region. We have confirmed that the cloned DNA regions contain the mtr gene sequence by restriction fragment length polymorphism mapping and have determined that the cloned sequence codes for a messenger RNA transcript of approximately 1200 nucleotides in length.