POSITIONING OF NUCLEI IN THE SECONDARY MYCELIUM OF SCHIZOPHYLLUM-COMMUNE IN RELATION TO DIFFERENTIAL GENE-EXPRESSION

In this paper we propose a novel type of gene regulation in the MATA n ot equal MATB not equal heterokaryon of Schizophyllum commune by means of differential positioning of the nuclei. It was found that binuclea te hyphae with juxtaposed nuclei secrete SC4 hydrophobin (abundant dur ing fruit-body formation), while SC3 (abundant during aerial hyphae fo rmation in both mono- and dikaryons) appeared to be absent. Certain gr owth conditions disrupted the binucleate state in that the compatible nuclei became separated at a considerable distance. Under these condit ions SC4 was not secreted while SC3 was secreted to a high degree. Dis ruption of the binucleate state was earlier observed in developing aer ial hyphae which secrete SC3. Apparently when the nuclei are in close proximity the dikaryon-expressed genes are switched on by interaction of the products of the MATA and MATB mating-type genes, while SC3 is s uppressed by interacting products of the MATB genes, as occurs in the common MATA heterokaryon (MATA = MATB not equal). Growth conditions th at lead to disruption of the binucleate state apparently result in abo lishment of interaction between the MATB mating-type genes, Under thes e conditions, dikaryon-specific mRNAs do not accumulate in the MATA no t equal MATB not equal heterokaryon, while SC3 mRNA becomes highly abu ndant. (C) 1998 Academic Press.