In vitro translation and characterization of a unique histidine-rich protein mRNA in the avian malaria parasite Plasmodium lophurae.

Abstract

The histidine-rich protein (HRP) of the avian [white Pekin ducklings] malaria parasite P. lophurae contains 70% histidine. It was found in dense cytoplasmic granules and during the erythrocytic cycle it accumulates to represent 10% of the dry weight of the parasite. In the present work the HRP mRNA was studied by in vitro translation and by the use of a polyhistidine oligonucleotide probe. The HRP mRNA contained 2000-2100 nucleotides encoding a protein with an apparent MW of 50,000. A HRP of MW 35,000-40,000 was also produced in vitro, probably as a result of proteolytic cleavage of the MW 50,000 polypeptide which corresponds to in vivo labeled and purified HRP. The HRP represents a much larger proportion of the in vitro products synthesized in the homologus cell-free system compared to the rabbit reticulocyte system, and it reflected more closely the pattern of protein synthesis seen in vivo. HRP mRNA was more abundant in polysomes isolated from young parasites than in polysomes from mature schizonts. The HRP may accumulate as a result of amplified translation of its mRNA at certain stages of its erythrocytic cycle.