Figure 2.

Secondary structure of psaA-i1 and psaA-i2 intron RNAs from C. reinhardtii. The three exons of the psaA gene and tscA locus of C. reinhardtii are transcribed independently. Two group II introns are formed at the exon boundaries of psaA by RNA base pairing. The mature psaA mRNA is generated by two trans-splicing reactions. Both group II introns have a characteristic group II intron structure, comprising six helical domains (DI-DVI), which surround a central core. psaA-i1 is a tripartite intron, whereas psaA-i2 is a dipartite intron. Fragmentation sites are indicated by arrows. Abbreviations: EBS/IBS, exon-/intron-binding sites.