Structural features of the large subunit rRNA expressed in Plasmodium falciparum sporozoites that distinguish it from the asexually expressed subunit rRNA

Abstract

The developmentally regulated transcription of at least two distinct sets of nuclear-encoded ribosomal RNAs is detected in Plasmodium species. The identification of functional differences between the two sets of rRNAs is of interest. To facilitate the search for such differences, we have identified the 5.8S and 28S rRNAs from Plasmodium falciparum that are expressed in the sporozoite stage (S gene) of the parasites' life cycle in the mosquito host and compare them to transcripts expressed in the red blood cells (A gene) of the vertebrate host. This completes the first set of A- and S-type nuclear-encoded rRNA genes for a Plasmodium species. Analysis of the predicted secondary structures of the two units reveals the majority of differences between the A- and S-type genes occur in regions previously known to be variable. However, the predicted secondary structure of both 28S rRNAs indicates 11 positions within conserved areas that are not typical of eucaryotic rRNAs. Although the A-type gene resembles almost all eucaryotes, being atypical in only 4 of the 11 positions, the S gene is variant in 8 of the 11 positions. In three of these positions, the S-type gene resembles the consensus nucleotides for the 23S rRNA from Eubacteria and/or Archaea. A few differences occur in regions associated with ribosome function, in particular the GTPase site where the S-type differs in a base pair and loop from all known sequences. Further, the identification of compensatory changes at conserved points of interactions between the 5.8S-28S rRNAs indicates that transcripts from A- and S-units should not be interchangeable.