Organization of ribosomal RNA genes in Alternaria alternata Japanese pear pathotype, a host-selective AK-toxin-producing fungus.

DNA encoding ribosomal RNA (rRNA) of Alternaria alternata Japanese pear pathotype has been cloned in lambda replacement vector, lambda Fix. Restriction endonuclease mapping and Southern hybridization with the 18S and 28S rRNAs of Saccharomyces cerevisiae revealed the A. alternata rDNA to be tandemly repeating 8.15-kilobase pair unit. The restriction fragments of the unit were then subcloned in the plasmid vector Bluescribe M13- and partially sequenced. The determined sequences were compared with previously reported sequences of S. cerevisiae rRNAs and their genes. The locations of DNA sequences encoding the 5.8S, 18S, and 28S rRNAs were determined by homology search using reported sequences. The complete DNA sequence for 5.8S rRNA of the fungus was found to be highly conserved at more than 90% homology in the fungi analyzed. However, sequence diversities were observed in limited regions involved in a helix structure, the helix (e), found at position 116-137.