Abstract
The apicoplast, an organelle found in Plasmodium and many other parasitic apicomplexan species, is a remnant chloroplast that is no longer able to carry out photosynthesis. Very little is known about primary transcripts and RNA processing in the Plasmodium apicoplast, although processing in chloroplasts of some related organisms (chromerids and dinoflagellate algae) shows a number of unusual features, including RNA editing and the addition of 3′ poly(U) tails. Here, we show that many apicoplast transcripts are polycistronic and that there is extensive RNA processing, often involving the excision of tRNA molecules. We have identified major RNA processing sites, and have shown that these are associated with a conserved sequence motif. We provide the first evidence for the presence of RNA editing in the Plasmodium apicoplast, which has evolved independently from editing in dinoflagellates. We also present evidence for long, polycistronic antisense transcripts, and show that in some cases these are processed at the same sites as sense transcripts. Together, this research has significantly enhanced our understanding of the evolution of chloroplast RNA processing in the Apicomplexa and dinoflagellate algae.
Highlights
Some of the major drugs used for combatting malaria, such as the antibiotics doxycycline and clindamycin, target gene expression in the apicoplast, an organelle found in Plasmodium and other members of the Apicomplexa group of parasitic eukaryotes
We have shown that Plasmodium apicoplast transcripts are not polyuridylylated (Dorrell et al 2014)
We carried out cDNA synthesis using a reverse primer within the SSU sequence, and used the cDNA and the same reverse primer in a series of PCRs with multiple forward primers located at increasing intervals upstream of the SSU rRNA gene, at least as far as the trnV sequence
Summary
Some of the major drugs used for combatting malaria, such as the antibiotics doxycycline and clindamycin, target gene expression in the apicoplast, an organelle found in Plasmodium and other members of the Apicomplexa group of parasitic eukaryotes. Inhibition of apicoplast transcription and translation is lethal to the parasite, as shown by treatment by rifampicin (a transcription inhibitor), thiostrepton or doxycycline (translation inhibitors) (Goodman et al 2007). Northern blots using total Plasmodium RNA revealed that the transcription of at least some apicoplast genes is likely to be polycistronic, as the bands seen were larger than would be expected for a single-gene RNA molecule (Gardner et al 1991a, b). There are no recognisable eubacterial promoter elements upstream of apicoplast genes, so it is unclear how transcription is initiated (Sato 2011)
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