Species-specific regulation and switching of transcription between stage-specific ribosomal RNA genes in Plasmodium berghei

Malaria parasites (Plasmodium spp.) differentially express structurally distinct sets of rRNA genes in a stage-specific manner. The four rRNA genes of the rodent malaria parasite, P. berghei, form two classes of 2 units that are genetically unlinked and termed A-type and S-type. Through Northern analysis and in situ hybridization, expression of the units was demonstrated in synchronized parasite preparations covering the developmental pathway from the initiation of the blood-stage asexual cycle to the production of mature ookinetes. A-type units were transcribed in direct response to cell growth in bloodstage asexual parasites yet were differentially regulated during male (inactive) and female (active) gametocytogenesis. S-type expression was not confined solely to the mosquito stages and exhibited a finite period of expression in a subset of bloodstage trophozoites that was significantly elevated in gametocyte-producing parasites. Unlike in the human parasite, P. falciparum, there was no evidence for accumulation of precursor forms of the S-type transcripts in gametocytes. No significant rRNA transcription was observed in cultured, fertilized ookinetes until approximately 20 h of development when S-type transcription was initiated. The results further demonstrate that in Plasmodium the expression of the different rRNA units is linked to developmental progression but in a species-specific manner.