Transcription of the apicoplast genome

Many members of the Apicomplexa contain a remnant chloroplast, known as an apicoplast. The apicoplast encodes numerous genes, and loss of the organelle is lethal. Here, we present a summary of what is known about apicoplast transcription. Unlike plant chloroplasts, there is a single RNA polymerase, and initial transcription is polycistronic. RNA is then cleaved into tRNA, mRNA and rRNA molecules. Significant levels of antisense transcription have been reported, together with a single case of RNA editing. Polycistronic transcription is also observed in the related algae Chromera and Vitrella, which retain a photosynthetic chloroplast. Surprisingly, a polyU tail is added to Chromera and Vitrella transcripts which encode proteins involved in photosynthesis. No such tail is added to Plasmodium transcripts. Transcription in the Apicomplexa is remarkably similar to that seen in the chloroplast of the related peridinin dinoflagellate algae, reflecting the common evolutionary origins of the organelle.