Genomic organisation and developmentally regulated expression of an apicomplexan aspartyl proteinase

The cDNA for an aspartyl proteinase, termed eimepsin, was isolated from an Eimeria tenella sporulated oocyst library and the deduced amino acid sequence found to be almost identical to a previously described aspartyl proteinase from E. acervulina (97.4% amino acid identity). An E. tenella cosmid clone covering the entire eimepsin gene was cloned and characterised. Sequencing revealed that the eimepsin gene spans 2.9 kb and consists of 18 exons and 17 introns. The 5' flanking region sequence of the gene contains a putative transcriptional promoter sequence (TATAAA box) and three potential transcription initiator sites (Inr sites). Expression of eimepsin at the mRNA and protein level is developmentally regulated during oocyst sporulation. The eimepsin transcript was detected in unsporulated oocysts and increased in abundance during the early part of sporulation when the oocyst undergoes nuclear division and blast formation. Thereafter, the level of the eimepsin transcript decreases and in the excysted sporozoite, no eimepsin-specific RNA was detected. Expression of eimepsin lags behind transcript expression by some hours, and the protein accumulates in the oocyst during sporocyst and sporozoite formation.