Figure 2

Group I introns in extrachromosomal nuclear rDNA of myxomycetes. (A) The rDNA mini-chromosomes are located within the nucleolus of myxomycetes. The multicopy mini-chromosomes of Didymium iridis contain the SSU and LSU rRNA genes and have regular telomeres at the ends. The rRNA genes harbor three group I introns (yellow). The S956 intron in the SSU is self-splicing and contains HEG, while the two introns (L1949 and L2449) in the LSU are obligatory introns dependent on the host for splicing. In fact, two versions of D. iridis S956 have been described: the twin-ribozyme intron S956-1 in the Panama 2 isolate and S956-2 in the Costa Rica 8 isolate with antisense HEG orientation. (B) The location of the group I introns within the SSU and LSU are shown for the Diderma niveum Italian isolate and the Physarum polycephalum Carolina isolate. All species contain the obligatory introns L1949 and L2449. The 20 group I introns found in D. niveum have four main categories. P. polycephalum also contains a mobile intron at position L1925 encoding I-Ppo I. For nomenclature of rDNA introns and insertion sites, see [8]. HEG: homing endonuclease gene; LSU rRNA: large subunit ribosomal RNA; nt: nucleotide; rDNA: ribosomal DNA; SSU rRNA: small subunit ribosomal RNA.