Mutational Analysis of a Regulatory Gene for Morphogenesis in Schizophyllum

Abstract

Knowledge of the structure and function of a complex gene regulating sexual development in the mushroom, Schizophyllum commune, has come from the analysis of various mutations in a chromosomal region known as the BJ3 incompatibility gene. This gene is one of two linked genes, Ba and Bfl, that together comprise the B factor which regulates a developmental sequence known as the B-sequence. The B-sequence is normally "turned off" (Fig. 1 a-e), and each factor controls a part of the series (2). The Asequence comprises nuclear pairing (b), conjugate division (c), and formation of hook cell (c and d), and converges with the B-sequence in fusion of hook cell (e). The B-sequence consists of nuclear migration (a) and fusion of hook cells (e). Sexual development thus requires the operation of both Aand B-sequences, or A-on-B-on. The converse, no sexual expression in unmated haploid strains or in matings homozygous for A and B, is A-off-B-off.Profound biochemical alterations underlie the operation of the sequences and suggest shifts in the spectrum of genes expressed in the morphogenetic progression (3).The present report is concerned with regulatory activity of the B factor and the status of the B-sequence, B-off or B-on; consequently, the status of the A-sequence can be essentially ignored. The terms "off" and "on" as used here signify only the inactivity or operation of the developmental sequence; they are not meant to imply anything about the mode of action of the incompatibility factors.The two genes of the B factor, Ba and Bof, each with nine 1427 known alleles (4), appear to be functionally identical and to act independently. Thus, the matings B al-f#l x B al-#2, B al-#l x B a2-01, and B a3-#4 x B a5-36 lead to a common result: to "turn on" the B-sequence and the characteristic B-on morphology. Matings homozygous for a single B factor, e.g., B a3-f32 x B a3S-#2 fail to turn on the B-sequence, and the phenotype remains B-off.The manner in which the genes of the B factor operate to control the B-sequence of sexual morphogenesis is unknown, but a study of the mutative capabilities provides some clues. All of many mutant alleles recovered in a system designed to select for a change from B-off to B-on were constitutive for the B-sequence (5, 6). The selective system would also have revealed mutations to new wild-type alleles capable of interacting with the progenitor allele to turn on the B-sequence, but no such mutation was found. Each of the mutants in isolation has a continuously operating B-sequence, e.g., the mutation of wild allele B,#2----+ B#2(1) (the number in parentheses representing a type of mutation) renders the B-sequence always on and makes the mutant allele interactive with all wild Bf3 alleles, the progenitor Bg2 included. The B-always-on phenotype is characterized by stunted growth, irregular branching, disrupted septa, and continuous nuclear migration; when the A-sequence is also operative, B-always-on elicits fusion of hook cells (7).Several secondary mutations induced in mutant B-on alleles ...

Basidiomycete Mating Type Genes and Pheromone Signaling

“…The most complex structure so far found is for the B␤2 allele, which codes for one receptor and eight pheromones, with three more pheromones probable from the genomic sequence (22). The B␤2 allele was of special interest due to the numerous primary and secondary mutations that had been induced earlier to show that reversion of tetrapolar to bipolar mating systems is possible and indicating multiple genes responsible for B mating type function (76,81,82). The primary mutations were screened for induction of B-regulated development, thus rendering the mutant functionally bipolar and indicating constitutive signaling without mating (76).…”

“…The primary mutations were screened for induction of B-regulated development, thus rendering the mutant functionally bipolar and indicating constitutive signaling without mating (76). Secondary mutations were then induced, suppressing the constitutive phenotype and returning phenotypically to a nonconstitutive situation not exactly the same as that of the wild type but associated with loss of some specificity in tetrapolar mating interactions (81,82). It was expected that the primary mutations should be in the receptor gene, rendering it constitutively active or responsive to (some) self-pheromones.…”

The pachytene karyotype of Schizophyllum commune analyzed by three dimensional reconstruction of synaptonemal complexes

“…Schizophyllum commune is serving as an object for studies of various genetic phenomena such as the evolution of a natural polymorphism (13), genetic regulation of morphogenesis (18,19), mitotic recombination (6), and repair of UV induced damage (12). In addition, phenomena directly related to meiosis are currently studied with this organism such as the genetic control of recombination (14,20,21,22) and the increased frequency of spontaneous mutations during meiosis (15).…”

A carboxy-subterminal aromatic residue in Schizophyllum commune mating pheromones controls specific recognition by Bar4 receptor

“…Each mutant pheromone has a single amino acid difference at the carboxy-subterminal amino acid position compared to its wild-type progenitor. A tryptophan auxotrophic strain of S. commune (V201-106, trp1 -, ΔB-MAT; FGSC#9350), which has no endogenous mating pheromone or receptor activity due to a large deletion in the B-MAT locus (Raper and Raper, 1973;Fowler et al, 1998), was co-transformed with two plasmids, one containing the pheromone gene to be tested and the second containing wild-type trp1 (pRHV1, Horton and Raper, 1995). Protoplasts were generated for transformation with Novozyme 234 and transformed by a PEG-mediated method (Specht et al;1988 Horton andRaper, 1991), using 20 ug of the pheromone gene plasmid and 10 ug of pRHV1.…”