Introns are necessary for mRNA accumulation in Schizophyllum commune

The cDNA coding sequence of the Agaricus bisporus hydrophobin gene ABH1 under the regulation sequences of the Schizophyllum commune SC3 hydrophobin gene gave no expression in S. commune. In contrast, the genomic coding sequence (containing three introns) produced high levels of ABH1 mRNA when transformed to S. commune in the same configuration. Apparently, introns were needed for the accumulation of mRNAs from the ABH1 gene. When the effect of intron deletion on expression of the homologous genes SC3 and SC6 was examined, it was observed that only the genomic coding sequences were expressed in S. commune. Run-on analysis with nuclei harbouring intron-containing and intronless SC6 showed that this effect did not occur at the level of transcription initiation: genomic and cDNA sequences were equally active in this respect. When a 50 bp artificial intron containing the consensus splice and branch sites of S. commune introns, in addition to random-generated sequences, was introduced in the right orientation into the intronless SC3 transcriptional unit, accumulation of SC3 mRNA was restored. By polymerase chain reaction amplification, no unspliced SC3 mRNA species could be detected. Furthermore, the addition of an intron into the transcriptional unit of the gene for green fluorescent protein (GFP) effected clear fluorescence of the transgenic hyphae. Apparently, splicing is required for the normal processing of primary transcripts in S. commune.