Functional characterization of the promoter and the gene product of pfg27, a Plasmodium falciparum gametocyte-specific gene

Malaria kills every year 1 to 3 million people, mostly children under five years old. The situation is becoming increasingly dangerous as drug resistant malaria parasites are diffusing throughout the world. Malaria transmission depends on the formation of Plasmodium gametocytes in the human host and methods to prevent sexual differentiation would help containing the spread of drug resistant parasites. This work concentrated on the poorly described early phases of gametocyte maturation, and particularly on the expression and the function of the gametocyte-specific protein Pfg27. This protein was reported to be essential for sexual development, as pfg27 gene disruption led to abolishment of parasite sexual differentiation. The genomic upstream region of pfg27 was characterized in functional assays through deletion and 5’UTR substitution analysis, with the aim to identify genomic sequences necessary for the pfg27 promoter function. These experiments identified the pfg27 promoter and showed that the DNA sequence sufficient to drive the stage-specific expression of the reporter gene is 153 bp long. The most striking feature of this sequence is its unusual low complexity, and the presence of poly-dA and poly-dT tracts, both around 40 bp long. A functional analysis on the large 3’UTR region of the gene was also performed, and indicated that this element contributes to protect the pfg27 transcript from decay. In order to investigate functional domains of the Pfg27 protein, we planned to introduce in a pfg27 knock-out strain constructs expressing wt and mutated forms of Pfg27. When such Ko strains were generated in our lab, a completely unexpected result was observed: parasites in which the Pfg27 protein was undetectable were still able to develop into gametocytes at levels comparable to the wild type parasites. Only ultrastructural analysis revealed that abnormal features in the subcellular structure were significantly more frequent in the Ko gametocytes than in the wt cells. For this reason, Pfg27 does not appear to be essential for gametocyte development, but it may be required for optimal production of sexual parasites.