The four ribosomal DNA units of the malaria parasite Plasmodium berghei. Identification, restriction map, and copy number analysis

The four ribosomal RNA genes (rDNA units) of the rodent malaria parasite, Plasmodium berghei, were identified and mapped by restriction enzyme analysis and Southern blot hybridization of genomic DNA. Although the four genes share common characteristics, they appear to be internally different from each other in expanse and sequence. One HindIII site near the 3' end of the coding region for the large rRNA is the only restriction site which we have detected that is conserved in all of the genes. The distance between the conserved HindIII site and the coding region for the small rRNA is at least 1-2 kilobases longer in two of the rDNA units than in a third unit. None of the four rDNA units were linked by restriction mapping where about 150 kilobases of the genome were accounted for. The copy number of two of the rDNA units was determined to be approximately 1 per haploid genome by quantitative analysis of cloned (plasmid) DNA versus genomic DNA digests on Southern blots. The other two genes also appear to be present in 1 copy. Restriction analysis confirms both the low copy number and that these genes are not in an easily recognizable tandem array. The low number of rDNA units requires that new copies of the genome produced during intraerythrocytic growth be active in RNA synthesis soon after their replication.