Rapid Changes in the Expression of a Gene Encoding a Calcium-binding Protein in Schistosoma Mansoni

Genes expressed in a stage-specific manner may help us understand the molecular events controlling the complex life cycle of schistosomes. cDNA and genomic clones encoding a calcium-binding protein (CaBP) were obtained from cercariae and their sequence determined. The encoded protein (69 amino acids long) shows clear resemblance to the domain structure and organization of CaBP molecules. It contains two typical calcium-binding loops, the distance between which is identical to the length conserved in other CaBP molecules. In addition, the schistosome CaBP shows Ca2+-dependent electrophoretic mobility (increased with Ca2+-ions and decreased with EGTA). Northern blots revealed expression of the CaBP gene in cercariae but not in sporocyst or worm (developmental stages preceding and following cercaria). The preferential expression of this CaBP in the cercaria raises questions as to what cercaria-specific function(s) it performs. The structure of the gene is similar to that in other eukaryotes, and one intron interrupts the coding sequence. The region of the cap site was determined, and there was no evidence of the spliced leader sequence found in the mRNAs of other parasites. The CaBP reveals a rapid change in gene expression, since the mRNA is missing in the parasite residing in infected snails, but is readily detected in cercariae 1 h after shedding. We identified other genes which are turned on (like the CaBP) or shut off within the short period of transition from cercariae in the snail to free-swimming cercariae.