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Abstract: The respective positions of the rDNA genes [5', 5S-18S-5.8S-25S, 3'] of Pleurotus cornucopiae were established by hybridization with cDNA probes synthezised from the rRNAs. The 5' location of the 5S gene contrasts with its 3' location in other basidiomycetes. The rDNA units were studied by RFLP analysis. The restrictions maps distinguished two types of rDNA units (types I and II). In each type, length variations of the intergenic sequence IGS 1 between the 25S and 5S genes, characterized six rDNA units : two of type I, and four of type II. This suggests that the P. cornucopiae units would be derived from two kinds of ancestors (type I and II) by insertion/deletion (100 b to 700 b) in the IGS 1. The rDNA organization in basidiomycetes is discussed.
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