STRUCTURAL FEATURES OF THE LARGE SUBUNIT RIBOSOMAL-RNA EXPRESSED IN PLASMODIUM-FALCIPARUM SPOROZOITES THAT DISTINGUISH IT FROM THE ASEXUALLY EXPRESSED LARGE SUBUNIT RIBOSOMAL-RNA

The developmentally regulated transcription of at least two distinct s ets of nuclear-encoded ribosomal RNAs is detected in Plasmodium specie s, The identification of functional differences between the two sets o f rRNAs is of interest, To facilitate the search for such differences, we have identified the 5,8S and 285 rRNAs from Plasmodium falciparum that are expressed in the sporozoite stage (5 gene) of the parasites' life cycle in the mosquito host and compare them to transcripts expres sed in the red blood cells (A gene) of the vertebrate host, This compl etes the first set of A- and S-type nuclear-encoded rRNA genes for a P lasmodium species, Analysis of the predicted secondary structures of t he 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 285 rRNAs indicates 11 posit ions within conserved areas that are not typical of eucaryotic rRNAs, Although the A-type gene resembles almost all eucaryotes, being atypic al 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 235 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 pai r and loop from all known sequences. Further, the identification of co mpensatory changes at conserved points of interactions between the 5,8 S-28S rRNAs indicates that transcripts from A- and S-units should not be interchangeable.