The white rot fungus Schizophyllum commune is used for the analysis of mating and sexual development in homobasidiomycete fungi. In this study, we isolated the gene gap1 encoding a GTPase-activating protein for Ras. Disruption of gap1 should therefore lead to strains accumulating Ras in its activated, GTP-bound state and to constitutive Ras signaling. Haploid ⌬gap1 monokaryons of different mating types did not show alterations in mating behavior in the four different mating interactions possible in fungi expressing a tetrapolar mating type system. Instead, the growth rate in ⌬gap1 monokaryons was reduced by ca. 25% and ca. 50% in homozygous ⌬gap1/⌬gap1 dikaryons. Monokaryons, as well as homozygous dikaryons, carrying the disrupted gap1 alleles exhibited a disorientated growth pattern. Dikaryons showed a strong phenotype during clamp formation since hook cells failed to fuse with the peg beside them. Instead, the dikaryotic character of the hyphae was rescued by fusion of the hooks with nearby developing branches. ⌬gap1/⌬gap1 dikaryons formed increased numbers of fruitbody primordia, whereas the amount of fruitbodies was not raised. Mature fruitbodies formed no or abnormal gills. No production of spores could be observed. The results suggest Ras involvement in growth, clamp formation, and fruitbody development.The homobasidiomycetous white-rot fungus Schizophyllum commune has been used as a model system for the investigation of mating and sexual development for decades since it can be grown from spore to spore through its entire life cycle within 14 days on defined media, and it shows easily distinguished phenotypes for a tetrapolar mating behavior (32, 53). The tetrapolar mating system consists of two sets of mating type genes. The A mating type genes encode homeodomain transcription factors that are assumed to directly regulate gene expression (39,67). A multiallelic pheromone/receptor system is encoded by the B genes. Homology to the yeast pheromone system has led to the expectation that mating is in part controlled by a mitogen-activated protein kinase signal transduction cascade that is activated after stimulation of the G protein-coupled pheromone receptor (20,76,82).A fully compatible mating between two strains of S. commune occurs when both differ in their A and B gene specificities (A B ). The specificity of a locus is defined by a lack of activation of downstream developmental processes after crossing two strains with identical specificities in their mating type genes (20). Several steps of subsequent development can be distinguished. After cell fusion, septal breakdown and fast nuclear migration allow reciprocal nuclear exchange between the two mates. Migrant and resident nuclei pair, and dikaryotic hyphal tips are established. Subsequent conjugate nuclear division is accompanied by formation of clamp connections. Clamp connections are short, backwardly directed branches that fuse with the subapical cell and provide a bypass for one of the nuclei produced during synchronous division of the dikaryon, en...

The genome of the model legume Medicago truncatula Gaertn. was screened for the presence of genes encoding tonoplast intrinsic proteins, and a gene family was identified. The cDNA fragments of two members of the multigene family were cloned from roots inoculated with an arbuscular mycorrhizal fungus. Transcript accumulation in roots could be detected for both cDNA fragments, but only one gene was induced in the symbiosis when compared to non-mycorrhizal control roots. A full-length cDNA clone was obtained from the arbuscular-mycorrhiza-regulated gene, and injection of in-vitro-transcribed RNA into Xenopus oocytes revealed that the encoded protein MtAQP1 specifically facilitates water transport. The possible role of MtAQP1 in buffering osmotic fluctations in the highly compartmented vacuole of arbuscule cells is discussed.

We have isolated 25 new strains of streptomycetes from soil samples of a polluted site at the former uranium mine, Wismut, in eastern Thuringia, Germany. The strains grew on medium containing 1 mM NiCl2 and thus were resistant to the heavy metal ion. Seven of the strains were further characterized. All of these strains were resistant to heavy metals in various degrees with up to 10 mM resistance against NiCl2 supplied with the liquid minimal growth medium. The high level of resistance prompted us to look for high affinity nickel transporter genes thought to provide a means to eliminate the excess nickel ions form the cells. Degenerate oligonucleotide primers derived from sequences of P‐type ATP‐ase transporter genes of Gram negative bacteria identified a fragment which shows deduced amino acid sequence similarities to known high affinity nickel transporters. Investigation of two genes obtained from the isolates Streptomyces spec. E8 and F4 showed high sequence divergence. This was unex‐pected since a transmissible plasmid had been thought to convey heavy metal resistance.

The white-rot fungus Schizophyllum commune (Agaricomycetes) was used to study the cell biology of microtubular trafficking during mating interactions, when the two partners exchange nuclei, which are transported along microtubule tracks. For this transport activity, the motor protein dynein is required. In S. commune, the dynein heavy chain is encoded in two parts by two separate genes, dhc1 and dhc2. The N-terminal protein Dhc1 supplies the dimerization domain, while Dhc2 encodes the motor machinery and the microtubule binding domain. This split motor protein is unique to Basidiomycota, where three different sequence patterns suggest independent split events during evolution. To investigate the function of the dynein heavy chain, the gene dhc1 and the motor domain in dhc2 were deleted. Both resulting mutants were viable, but revealed phenotypes in hyphal growth morphology and mating behavior as well as in sexual development. Viability of strain Δdhc2 is due to the higher expression of kinesin-2 and kinesin-14, which was proven via RNA sequencing.

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