Cloning and characterization of a ribosomal RNA gene from Plasmodium berghei

The ribosomal RNA (rRNA) of Plasmodium berghei strain NYU2 was characterized with respect to size and used to identify molecular clones of ribosomal RNA genes (rDNA) from this organism. The intact large rRNA species (Mr 1.40 X 10(6] is intermediate in size between prokaryotic and eukaryotic large rRNAs. It is specifically cleaved in vivo near the 5'-end yielding one large component (Mr 1.10 X 10(6] and one small component (Mr 0.30 X 10(6]. The small rRNA species (Mr 0.75 X 10(6] is larger than in higher eukaryotic organisms, but is similar in size to some other protozoa. Using 5'-end labeled rRNA as a probe, clones of rDNA were selected from genomic DNA libraries prepared in lambda phage Charon 4A and plasmid pBR322. The clones prepared in lambda were prone to recombination. The 14.7 kilobase (kb) insert in phage lambda PbR27 contains the complete coding sequences for both the small and larger rRNA species. A complete set of small plasmid clones were isolated which hybridize to the different segments of the rRNA transcription unit. A restriction map of the genes cloned in lambda PbR27 with the approximate position of the coding region is presented. The polarity of transcription of the rDNA was determined by hybridizing 3'-OH end labeled rRNA to Southern blots of genomic and cloned rDNA. The results indicate that a transcription unit of P. berghei rDNA is arranged as in other organisms with the small rRNA at the 5' end and the large rRNA at the 3' end.