Chimeric rRNAs containing the GTPase centers of the developmentally regulated ribosomal rRNAs of Plasmodium falciparum are functionally distinct IRINA V. VELICHUTINA, 1 M. JOHN ROGERS, 2,3 THOMAS F. McCUTCHAN, 2 and SUSAN W. LIEBMAN 1 1 Laboratory for Molecular Biology, Department of Biological Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, USA 2 Growth and Development Section, Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-0425, USA ABSTRACT The human malaria parasite, Plasmodium falciparum, maintains at least two distinct types, A and S, of developmen- tally controlled ribosomal RNAs. To investigate specific functions associated with these rRNAs, we replaced the Saccharomyces cerevisiae GTPase domain of the 25S rRNA with GTPase domains corresponding to the Plasmodium A- and S-type 28S rRNAs. The A-type rRNA differs in a single nonconserved base pair from the yeast GTPase domain. The S-type rRNA GTPase domain has three additional changes in highly conserved residues, making it unique among all known rRNA sequences. The expression of either A- or S-type chimeric rRNA in yeast increased translational accuracy. Yeast containing only A-type chimeric rRNA and no wild-type yeast rRNA grew at the wild-type level. In contrast, S-type chimeric rRNA severely inhibited growth in the presence of wild-type yeast rRNA, and caused lethality in the absence of the wild-type yeast rRNA. We show what before could only be hypothesized, that the changes in the GTPase center of ribosomes present during different developmental stages of Plasmodium species can result in fundamental changes in the biology of the organism. Keywords: development; malaria parasite; ribosome; Saccharomyces cerevisiae; translation INTRODUCTION The human malaria parasite Plasmodium falciparum is unusual in having a low number of structurally distinct sets of ribosomal RNA genes located on different chro- mosomes (Wellems et al+, 1987), although the arrange- ment of this rDNA as an 18S–5+8S–25S unit is that of a typical eukaryote+ The unique feature of these Plas- modium gene sets is their stage-specific expression during the parasite life cycle (Gunderson et al+, 1987; Waters et al+, 1989; Li et al+, 1994a)+ P. falciparum is an obligate intracellular parasite that infects erythrocytes during the asexual stages of development (schizogony), which results in the clinical symptoms of malaria+ A-type rRNA is detected in asexual (blood) stages+ The para- site is transmitted by the bite of an anopheline mos- quito, where a motile form of the parasite (sporozoite) is injected into the peripheral blood supply+ S-type rRNA is detected in sporozoites purified from the salivary glands of infected mosquitoes+ Sporozoites migrate rap- idly to the liver following the bite of an infected mos- quito, where development takes place in hepatocytes to complete the cycle+ At least two distinct types (A- and S-type) of rRNA are therefore detected, correspond- ing to distinct stages of the parasite’s life cycle+ The switch from A- to S-type gene expression can be mon- itored during development of the parasite in the mos- quito (Li et al+, 1994b), and the corresponding S- to A-type transition in the liver has been found during development of sporozoites in a cultured hepatocyte cell line (Zhu et al+, 1990)+ This suggests that switches in rRNA gene expression are a critical feature during development in the life cycle of the malaria parasite+ For most Plasmodium species, the overall sequence similarity between the A- and S-type rRNA genes is Reprint requests to: S+W+ Liebman, University of Illinois at Chi- cago, Department of Biological Sciences, Molecular Biology Re- search Building, 900 South Ashland Avenue, Room 4070, Chicago, Illinois 60607, USA; e-mail: suel@uic+edu+ 3 Present address: Antimicrobial Group, DuPont Merck Pharma- ceutical Company, Wilmington, Delaware 19880-0400, USA+ RNA (1998), 4:594–602+ Cambridge University Press+ Printed in the USA+ Copyright © 1998 RNA Society+ 594