Expression of the cercosporin toxin resistance gene (CRG1) as a dicistronic mRNA in the filamentous fungus Cercospora nicotianae

The CRG1 gene in Cercospora nicotianae encodes a transcription factor and is required for cercosporin toxin resistance and production. Cloning and sequencing of the downstream region of the CRG1 gene led to the discovery of an adjacent gene (PUT1) encoding a putative uracil transporter. Expression of CRG1 and PUT1 as assessed by Northern analysis indicated that, in addition to the expected monocistronic mRNAs (2.6 kb and 2.0 kb, respectively), a common 4.5-kb mRNA could be identified, using either a CRG1 or a PUT1 gene probe. The 2.6-kb transcript identified only by the CRG1 probe was expressed constitutively, whereas the 2.0-kb transcript identified only by the PUT1 probe was differentially expressed in various media. Four cDNA clones containing CRG1, PUT1, and the CRG1-PUT1 intergenic region were identified as part of the products from the 4.5-kb transcript. Both the 4.5-kb and 2.6-kb transcripts were not detectable in three crg1-disrupted mutants, using the CRG1 probe. The 2.0-kb transcript, but not the 4.5-kb one was detected using the PUT1 probe in the three crg1-disrupted mutants. Taken together, we conclude that the 4.5-kb transcript is a dicistronic mRNA of both CRG1 and PUT1 in the fungus C. nicotianae. This is the first example of a dicistronic mRNA identified in filamentous fungi.