Characterisation of a plastid DNA replicase in Plasmodium falciparum

Human malaria is caused by the parasite genus, Plasmodium, with P. falciparum being the most pathogenic species. The disease affects between 300 to 500 million worldwide. Currently, efforts to contain the disease are curbed by the limited range of antimalarials and the rise of resistance to these treatments. The race is now on to gain a thorough understanding of the biology of the parasite that can be use for the development of strategies to combat the wrath of this disease. Since the 22.8 Mb genome sequence of P. falciparum has been fully sequenced, the data has provided new candidate genes that may be valid drug targets. One area that is of great interest is that of the apicoplast of Plasmodium. The evolutionary origin of the apicoplast is generally believed to reflect a secondary endosymbiotic event, in which a proto-eukaryote harbouring a photosynthetic cyanobacterium itself became an endosymbiont of a heterotrophic eukaryote. In Plasmodium, this organelle appears to be essential in these parasites and enzyme systems specific to apicoplast have been identified as targets for anti protozoal chemotherapy. Segments of a gene (PfPREX-, formerly referred to as PfPOMl), encoding a large protein with a typical N-terminal bipartite plastid targeting sequence have been cloned from Plasmodium falciparum and expressed in bacteria. Domains with homology to prokaryotic DNA polymerase, DNA primase and DNA helicase, all were found specified in an opening reading frame (ORF). The putative polymerase domain possesses both DNA polymerase activity and 3'-5' exonuclease activity. The putative helicase-primase domain has also been expressed and both DNA primase and helicase activities were detected. RT-PCR indicates that the open reading frame is expressed as a single transcript whilst Western blot analysis has shown that the protein is cleaved post-translationally. Localisation studies carried out by collaborators (Sato S, NIMR, Mill Hill) with a GFP-reporter revealed that the protein is targeted exclusively to the plastid. In addition, the evolutionary history of PfPREX appears to be more complex and cannot be explained fully using the secondary endosymbiotic theory. The DNA polymerase domain has great homology to prokaryotic DNA polymerase I. The DNA primase and DNA helicase domain, on the other hand, shows striking similarity to proteins with these functions encoded in T-odd bacteriophages. T-odd bacteriophage related RNA polymerase proteins have previously been identified as playing roles in transcription of plastid genes. Now it seems that additional phage-like proteins also play a key role in the replication of the Plasmodium plastid genome. The presence of these T odd genes in Plasmodium may be explained with different theories, mainly the non-orthologous gene displacement with the T-odd bacteriophage genes. Nonetheless, the discovery of PfPREX has opened insights to many exciting possibilities and insight into Plasmodium falciparum. This gene can provide not only a valid drug target, due to its prokaryotic origin, but also an insight into the complex evolutionary history of Plasmodium