Developmental regulation of stage-specific ribosome populations in Plasmodium.

The Plasmodium parasites are so far unique in biology in possessing developmentally regulated ribosomal RNA gene units. Two different genes encode their small subunit rRNAs: one gene (A) yields transcripts predominant in the asexual blood-stage parasites, and the other (C) is mainly transcribed in the sporozoite forms that develop in the mosquito. Developmental control of events allowing a switch in the complement of ribosomes must coordinate the production of the new class with selective inactivation and removal of the old. We show here that in P. falciparum the switch, from A to C gene expression involves the control of rRNA processing, allowing accumulation of precursor C-gene transcripts in gametocytes. These precursor molecules are processed to mature size in the zygote and the early ookinete, where both transcription and processing of the C-gene rRNA seem to be accelerated. As the C-gene precursor rRNA appears, a defined and limited pattern of breakdown of the dominant A-gene rRNA occurs, in which conserved, functionally active sequences involved in the termination of translation and elongation are targeted. By the late oocyst stage, the A-gene transcripts are virtually replaced by mature C-gene transcripts.