Functional equivalence of structurally distinct ribosomes in the malaria parasite, Plasmodium berghei

Unlike most eukaryotes, many apicomplexan parasites contain only a few unli nked copies of ribosomal RNA (rRNA) genes. Based on stage-specific expressi on of these genes and structural differences among the rRNA molecules it ha s been suggested that Plasmodium spp. produce functionally different riboso mes in different developmental stages. This hypothesis was investigated thr ough comparison of the structure of the large sub unit rRNA molecules of th e rodent malaria parasite, Plasmodium berghei, and by disruption of both of the rRNA gene units that are transcribed exclusively during development of this parasite in the mosquito (S type rRNA gene units). In contrast to the human parasite, Plasmodium falciparum, we did not find evidence of structu ral differences in core regions of the distinct large subunit rRNAs which a re known to be associated with catalytic activity including the GTPase site that varies in P. falciparum. Knockout P. berghei parasites lacking either of the S-type gene units were able to complete development in both the ver tebrate and mosquito hosts. These results formally exclude the hypothesis t hat two functionally different ribosome types distinct from the predominant ly blood stage-expressed A-type ribosomes, are required for development of all Plasmodium species in the mosquito. The maintenance of two functionally equivalent rRNA genes might now be explained as a gene dosage phenomenon.