Visualization of Mitochondrial and Apicoplast Nucleoids in the Human Malaria Parasite Plasmodium falciparum by SYBR Green I and PicoGreen Staining

Organellar DNA in mitochondria and plastids are organized with proteins into a compact structure known as the nucleoid. As the nucleoid is supposed to be the unit of inheritance for the organellar genome, it is important to understand its cytological behavior. Like plants, malaria parasites carry two organelles, the mitochondrion and the apicoplast-a non-photosynthetic plastid. However, probably because of the small size of the genome in each, visualizing the nucleoid in the Plasmodium organelles by regularly-used fluorescent dye such as DAPI, has been difficult. Here, we developed new, effective methods to visualize the organellar nucleoid in the human malaria parasite Plasmodium falciparum. With our methods using SYBR Green I or PicoGreen, nucleoids were observed in ring-stage parasites. Analyzing transfectant parasites carrying a DsRed-labelled organelle, we concluded that the parasite's mitochondrion has 1 nucleoid which is visualized with our methods. The parasite has a second nucleoid in the apicoplast, but higher concentration of the dye was required to visualize it. Our new methods would be useful for further cytological analysis of the nucleoids in the mitochondrion and the apicoplast of the malaria parasite.