Heterogeneous ribosome populations are present in Plasmodium berghei during development in its vector

The genome of the rodent malaria parasite, Plasmodium berghei, contains two sets of variant ribosomal RNA (rRNA) genes, termed the A and S types, that are expressed predominantly during the vertebrate and mosquito stages of t he parasite's development respectively. Using in situ hybridization, we hav e examined the transcriptional activity of the A- and S-type rRNA genes, an d the switch in expression of the ribosome populations that occurs after pa rasite transmission to the mosquito. By detection of precursor rRNA molecul es, we show that A-type rRNA transcription is downregulated throughout deve lopment in the mosquito, whereas the initiation of S-type rRNA transcriptio n is linked to the proliferative phase of the oocyst. Mature A-type rRNA pe rsists during development of the zygote into the ookinete/young oocyst. In contrast, mature S-type rRNA is first detectable in young oocysts and is su bsequently present at high levels during further development of oocysts and sporozoites. These results demonstrate that the switch in transcription be tween the A- and S-type rRNA genes is developmentally regulated, taking pla ce only as the parasite begins to proliferate in the mosquito. A-type ribos omes are therefore not only translationally active in the early stages of d evelopment in the mosquito, but are also crucial at this phase.