Fig. 1.

Dispersed rRNA genes clustered in a single perinuclear nucleolus. (A) 3D7 P. falciparum linear chromosome map showing the positions of rRNA genes: five rDNA units (circles) located on chromosomes 1, 5, 7, 11, and 13, and a tandem of three 5S rRNA genes (stars) on chromosome 14. rDNA units from chromosomes 1, 5 and 7 correspond to the previously named S1, A2, and A1 genes, respectively (9, 11). rDNA copies from chromosomes 11 and 13 correspond to a duplication of the S2 gene. This duplication was confirmed by bioinformatic analysis and pulse-field gradient gel electrophoresis. A-types are transcribed in asexual blood stages, whereas S-types are predominantly transcribed in sexual stages of the parasite (8, 9, 11). The malaria genome project has identified two additional incomplete units (lacking 18S gene) on both subtelomeres of chromosome 8 (not indicated in map). It is not known whether these units are functionally active (6). Small vertical black bars denote centromere positions on each chromosome. Triangles indicate target locations of FISH probes used in this study (green in this figure and red in Fig. 3). (B) Immunofluorescence analysis using antibodies against the nucleolar protein PfNop1 (red) combined with DNA FISH (green) for individual rRNA genes (A1, A2, S1, S2, and 5S) and a subtelomeric unrelated gene (PFD0110w) on chromosome 4. The percentage of signals that localized in or near the nucleolus and the number of nuclei analyzed are as follows: A1 93.2%, n = 73; A2 90.5%, n = 95; S1 92.3%, n = 91; S2 90.0%, n = 80; 5S 90.6%, n = 128; PFD0110w 24.6%, n = 57. Parasites are in ring stage, and nuclear DNA is stained with DAPI (blue). (Scale bar, 1 μm.)