Secondary structure model of the Scenedesmus psaA intron. Intron modelling was according to the nomenclature proposed by Michel et al. . Exon sequences are shown in lowercase letters. Roman numerals specify the six major structural domains of group II introns. Tertiary interactions are represented by blocked arrows. EBS and IBS refer to exon-binding and intron-binding sites, respectively. Numbers inside the loops denote the sizes of these regions. The 5' and 3' strand polarities of the psaA a and psaA b transcripts are indicated by arrows.