Inversion of 5S ribosomal RNA genes within the genus Coprinus

In the basidiomycete fungus Coprinus cinereus, the 5S ribosomal RNA (rRNA) genes are found in the rDNA repeat which encodes the other three rRNAs (18S, 5.8S, and 26S), and all genes are transcribed in the same direction (Cassidy et al. 1984). To facilitate examination of the rRNA gene organization in additional species, we developed a method to permit determination of the direction of transcription of the rRNA genes using crude preparations of genomic DNA. We used this method to determine the organization of the rDNA in five other members of the order Agaricales (Flammulina velutipes, Agaricus bisporus, Coprinus micaceus, Coprinus atramentarius, and Coprinus comatus). In four, the organization is the same as it is in Coprinus cinereus. However, in Coprinus comatus, the 5S RNA gene is present in the rDNA repeat, but is transcribed in the opposite direction. This alteration in rDNA organization within the genus Coprinus indicates that inverted 5S genes can arise and become propagated through the tandem array of genes. The presence of orientation-dependent promoter elements within the spacer DNA (Elion and Warner 1984) would be expected to place constraints on such inversions.