Intron-encoded open reading frame of the GIY-YIG subclass in a plastid gene

Group-I introns, containing open reading frames (ORFs) that code for homing endonucleases, are widely distributed amongst eukaryotic organellar genomes. However, endonucleases of the GIY-YIG subclass have a restricted distribution in mitochondria and bacteriophages, and have never been observed in plastids. We have found the GIY-YIG motif in an intronic ORF within the previously published psbA gene sequence from Chlamydomonas reinhardtii chloroplasts. Based on phylogenetic analysis and an evaluation of amino-acid substitutions, this ORF is not closely related to any of the other GIY-YIG ORFs. These results suggest that GIY-YIG ORFs have a longer evolutionary history than previously assumed.