The intron of a plastid gene from a green alga contains an open reading frame for a reverse transcriptase-like enzyme

Plastid (pt) and mitochondrial (mt) DNA were isolated from the unicellular green alga KS3/2, which is presumed to be a species of the genera Ankistrodesmus or Monoraphidium. The DNA species are characterized by their different densities (pt, 1.685 g/ml; mt, 1.695 g/ml), individual restriction patterns, and their respective sizes of 130 and 47 kb. Using an intronic sequence from fungal mitochondria as a hybridization probe, intron sequences from mtDNA and from ptDNA were identified. DNA sequencing of the cloned ptDNA intron revealed that the plastid gene for subunit IV of the cytochrome b6/f petD complex is interrupted by a group II intron of unusual length (3533 bp). This was confirmed by transcript analysis. The intron encodes an open reading frame (ORF) which shows significant homology with reverse transcriptase genes from various genetic elements. This discovery is unique for plastomes and indicates that introns, retrotransposons, insertion elements and retroviruses may have a common evolutionary origin.