Exploring the transcriptome of the malaria sporozoite stage

Most studies of gene expression in Plasmodium have been concerned with asex ual and/or sexual erythrocytic stages. Identification and cloning of genes expressed in the preerythrocytic stages lag far behind. We have constructed a high quality cDNA library of the Plasmodium sporozoite stage by using th e rodent malaria parasite A yoelii, an important model for malaria vaccine development. The technical obstacles associated with limited amounts of RNA material were overcome by PCR-amplifying the transcriptome before cloning. Contamination with mosquito RNA was negligible. Generation of 1,972 expres sed sequence tags (EST) resulted in a total of 1,547 unique sequences, allo wing insight into sporozoite gene expression. The circumsporozoite protein (CS) and the sporozoite surface protein 2 (SSP2) are well represented in th e data set. A BLASTX search with all tags of the nonredundant protein datab ase gave only 161 unique significant matches (P(N) less than or equal to 10 (-4)), whereas 1,386 of the unique sequences represented novel sporozoite-e xpressed genes. We identified ESTs for three proteins that may be involved in host cell invasion and documented their expression in sporozoites. These data should facilitate our understanding of the preerythrocytic Plasmodium life cycle stages and the development of preerythrocytic vaccines.