Intron-rich Gene Structure in the Intracellular Plant Parasite Plasmodiophora brassicae

Plasmodiophora brassicae, a pathogen of Brassicaceae plants, is grouped within the eukaryotic supergroup, the Rhizaria. Although a large diversity of protists is found in the Rhizaria, genomes of organisms within the group have barely been examined. In this study, we identified DNA sequences spanning or flanking 24 P. brassicae genes, eventually sequencing close to 44 kb of genomic DNA. Evidence from this preliminary genome survey suggested that splicing is an important feature of P. brassicae gene expression; the P. brassicae genes were rich in spliceosomal introns and two mini-exons of less than 20 bp were identified. Consensus splice sites and branch-point sequences in P. brassicae introns were similar to those found in other eukaryotes. Examination of the promoter and transcription start sites of genes indicated that P. brassicae transcription is likely to begin from initiator elements rather than TATA-box containing promoters. Where neighbouring genes were confirmed, intergenic distances were short, ranging from 44 to 470 bp, but a number of larger DNA fragments containing no obvious genes were also sequenced.