Multiple Splice Variants Encode a Novel Adenylyl Cyclase of Possible Plastid Origin Expressed in the Sexual Stage of the Malaria Parasite Plasmodium falciparum

Abstract

We report the characterization of an unusual adenylyl cyclase gene from Plasmodium falciparum, here designated PfACalpha. The level of mRNA expression is maximum during development of gametocytes (the sexual blood stage of the parasite life cycle). The gene is highly interrupted by 22 introns, and reverse transcriptase-PCR analysis revealed that there are multiple mRNA splice variants. One intron has three alternative 3'-splice sites that confer the potential to encode distinct forms of the enzyme using alternative start codons. Deduced amino acid sequences predict membrane-spanning regions, the number of which can vary between two and six depending on the splice variant. Expression of a synthetic form of two of these variants in Xenopus oocytes and in Dictyostelium adenylyl cyclase-deficient mutants, confirms that PfACalpha is a functional adenylyl cyclase. These results identify a novel mechanism in P. falciparum for the generation of multiple isoforms of a key, membrane-bound signaling molecule from a single genomic copy. Comparisons of the catalytic domains of PfACalpha and a second putative P. falciparum adenylyl cyclase (PfACbeta) with those from other species reveal an unexpected similarity with adenylyl cyclases from certain prokaryotes including the cyanobacteria (blue green algae). In addition, the presence of an unusual active site substitution in a position that determines substrate specificity, also characteristic of these prokaryotic forms of the enzyme, further suggests a plastid origin for the Plasmodium cyclases.