Fig. 4

Structural organization of L2066 group I intron in Didymium squamulosum. a Secondary structure diagram of the Costa Rican (CR10) isolate. Antisense homing endonuclease gene (HEG) insertion is indicated. See legend to Fig. 1a for structural annotations. b Schematic presentation of HEG insertion in P9. Start and stop codons are indicated, as well as the 47 nt spliceosomal intron (I-47). Two AAUAAA polyadenylation signals are located close to the termination codon. Below is an alignment of selected spliceosomal introns in group I intron HEGs. I-47, Didymium squamulosum L2066; I-49, Diderma alpinum L2066; I-50, Didymium iridis S956–2; I-51, Didymium iridis S956–1. Yeast spliceosomal consensus sequences at branch and splice sites are indicated above the alignment. (*) indicate hallmark spliceosomal intron positions (GU-A-AG). c Schematic presentation of the I-DsqI homing endonuclease containing a His-Cys box. Below is an amino acid alignment of His-Cys box features in Didymium squamulosum (I-DsqI), Diderma alpinum (I-DalI), Didymium iridis (I-DirII), Naegleria jamiesoni (I-NjaI) and Physarum polycephalum (I-PpoI). I-NjaI and I-PpoI represent well-studied His-Cys homing endonucleases. Conserved residues (boxed) corresponding to those presented in the I-PpoI crystal structure [20]. C100, C105, H110 and C125 are involved in zinc binding, and H98 and N119 are associated with the active site