The clp1 gene of the mushroom Coprinus cinereus is essential for A-regulated sexual development

Sexual development in the mushroom Caprinus cinereus is under the control o f the A and B mating-type loci, both of which must be different for a compa tible, dikaryotic mycelium to form between two parents. The A genes, encodi ng proteins with homeodomain motifs, regulate conjugate division of the two nuclei from each mating partner and promote the formation of clamp connect ions. The latter are hyphal configurations required for the maintenance of the nuclear status in the dikaryotic phase of basidiomycetes. The B genes e ncode pheromones and pheromone receptors. The regulate the cellular fusions th;it complete clamp connections during growth, as well as the nuclear mig ration required for dikaryosis. The AmutBmut strain (326) of C. cinereus, i n which both A- and B-regulated pathways are constitutively activated by mu tations, produces, without mating, dikaryon-like, fertile hyphae with clamp connections. In this study we isolated and characterized clampless1-1 (clp 1-1), a mutation that blocks clamp formation, an essential step in A-regula ted sexual development, in the AmutBmut background. A genomic DNA fragment that rescues the clp1-1 mutation was identified by transformations. Sequenc ing of the genomic DNA, together with RACE experiments, identified an ORF i nterrupted by one intron, encoding a novel protein of 365 amino acids. The clp1-1 mutant allele carries a deletion of four nucleotides, which is predi cted to cause elimination of codon 128 and frameshifts thereafter. The clp1 transcript was normally detected only in the presence of the A protein het erodimer formed when homokaryons with compatible A genes were mated. Forced expression of clp1 by promoter replacements induced clamp development with out the need for a compatible. A gene combination. These results indicate t hat expression of clp1 is necessary and sufficient for induction of the A-r egulated pathways that leads to clamp development.