SUBREPEAT STRUCTURE OF THE INTERGENIC REGION IN THE RIBOSOMAL DNA OF THE OOMYCETOUS FUNGUS PYTHIUM-ULTIMUM

Restriction endonuclease site maps of the rDNA repeat in 14 isolates of the oomycetous fungus Pythium ultimum show that the intergenic region (IGR) includes two regions of heterogeneity. A region located about one kilobase downstream of the 3′ end of the large subunit rRNA gene contains a segment present in multiple versions differeing in size by as much as 0.9 kb. Each isolate shows a unique pattern of heterogeneity in this region. The other region of heterogeneity is located near the centre of the IGR and consists of segments which differ by integral multiples of 385 bp. Thus, the IGR in P. ultimum is dominated by an array of uniform subrepeats. The size of the repeat segment is identical in all isolates, but the number of subrepeats (6-12), and the relative abundance of each variant segment is highly polymorphic. Because no other fungus has been reported with this type or organization in its rDNA repeat, these findings support the idea that Oomycetes are phylogenetically distinct from other fungi. They also provide a new system for the study of the role of rDNA subrepeats in transcriptional regulation. © 1990 Springer-Verlag.