The RNA-binding KH-domain in the unique transcription factor of the malaria parasite is responsible for its transcriptional regulatory activity.

To date, only a small number of regulatory transcription factors have been predicted from the genome of Plasmodium and Apicomplexan parasites. We previously identified a nuclear factor named Prx regulatory element-binding protein (PREBP) from Plasmodium falciparum. PREBP had been suggested to bind to the cis-element in the promoter of an antioxidant pf1-cys-prx gene, thereby promoting the expression of downstream genes. PREBP has 4 putative K homology (KH) domains, which are known to bind RNA and single-stranded DNA. In this study, to understand the detailed action of PREBP in parasite cells, we first observed that in living parasite cells, PREBP was localized in the nucleus in the trophozoite and schizont stages, in which the expression of the target pf1-cys-prx was enhanced. The interaction of PREBP and the cis-element of pf1-cys-prx in the parasite cells was also confirmed. Further, the activities of PREBP deletion mutants were analyzed, and regions with repeated KH domains in PREBP seemed to be responsible for the recognition of the cis-element. These results led us to hypothesize that Plasmodium and other Apicomplexan parasites might have a transcription factor family with KH domains. Bioinformatic analysis revealed a putative ortholog group including PREBP and several Plasmodium and Apicomplexan factors with KH domains. One of the P. falciparum-derived factors, which were included in the putative ortholog group, was found to be localized at the nucleus in the trophozoite stage, indicating that it might be a novel transcription factor. The discovery of PREBP and putative transcription factors with KH domains suggested that multi-functional proteins with KH domains possibly evolved in the Apicomplexan organisms. They might play key roles in transcriptional regulatory processes that are essential for living organisms and may even represent unique drug targets for malaria therapy.