Molecular characterization of actin genes from homobasidiomycetes: two different actin genes from Schizophyllum commune and Suillus bovinus

The actin-encoding genes Scact1 and Scact2 of the homobasidiomycete Schizop hyllum commune are the first actin genes isolated from higher filamentous f ungi. Their isolation shows that homobasidiomycetes have two actin encoding genes instead of one typical to yeasts and filamentous ascomycetes. This r esult was further confirmed by cloning two actin encoding genes, Scact1 and Sbact2. from another homobasidiomycete Sullius bovinus. The comparison of the genomic and cDNA sequences of the actin genes showed that Scact1 and Sc act2 genes of S. commune contain seven introns, five of which are at the sa me position in the two genes while S. bovinus actin genes contain nine simi larly positioned introns. In the four genes, five intron positions are shar ed, which indicates a close relationship between the actin encoding genes f rom S. commune and S. bovinus. Northern hybridization and analysis of two-d imensional immunoblots showed a difference in the expression levels between the two actin genes in each fungus. No actin protein could be detected fro m S. commune Scact2. The deduced amino acid sequence of the Scact2 gene als o differs considerably from any other known actin protein. These data sugge st that the Scact2 gene either has a special as yet unidentified function i n S. commune life cycle or is a transcribed but no longer translated pseudo gene. Scact2 gene has a putative mu ORF (short open reading frame) and Scac t1 gene an intron in the 5'-untranslated region, which could reduce the tra nslational efficiency and increase the transcriptional efficiency of the Sc act2 and Scact1 genes, respectively. During mating in S. commune or at form ation of ectomycorrhiza in S. bovinus, the expression of actin genes was si milar to that in vegetative hyphae. This result suggests that the reorganiz ation of actin cytoskeleton in response to extra- and intracellular signals in higher filamentous fungi could be directly regulated by members of sign alling pathways well characterized in yeast and mammalian cells. (C) 2000 E lsevier Science B.V. All rights reserved.