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the P. reichenowi Msp-2 allelic repeats within this region. Although the conservation of these codons is clear among these two alleles, it appears that they have been lost altogether in the FC27-like alleles (represented by OKS in Fig. 5, Fig. 6, Fig. 7). However, the region adjacent to RHR1 in the P. reichenowi Msp-2 sequence is similar to the first 21 aa of the 32-aa repeat found within the FC27 family, and this sequence is the basis for the inferred RHR2 (Fig. 5, dark gray shading). The last 9 nt of RHR2 also manifest the homology between all three sequences, including the short stretch following the (actaccaa)4 repeat in 3D7. Note also the overlap between repeating nucleotides of P. reichenowi Msp-2 in both RHR1 and RHR2.

A third RHR is located further downstream and shows the relationship between the 12-aa repeats of OKS and P. reichenowi Msp-2 (Fig. 7). The repeat region in OKS is surrounded on either side by a 10-bp sequence (tacagaaagt), which occurs as only a single 5′ copy in the P. reichenowi Msp-2 allele. Despite the lengthy repeat insertion in the OKS sequence, the homology of OKS and the P. reichenowi Msp-2 in the region downstream of this repeat is apparent. And so it appears that the repeats were generated sometime after the split between P. falciparum and P. reichenowi.

Analysis of the single P. reichenowi sequence allows us to approximate the ancestral sequence of the two P. falciparum Msp-2 allele families. Indeed, the comparison of the three RHRs discloses that whereas the precursor sequences for the various repeats probably were derived from the common P. falciparum–P. reichenowi ancestral species, the extant diversity among the Msp-2 alleles has derived since the divergence of the two species. The distinctive dimorphism of the two P. falciparum alleles results from proliferation of repeats in two different regions of the molecule. Presumably because the overall MSP-2 molecule is constrained in size, the proliferation of repeats leads consequently to loss of nucleotides along the gene regions; i.e., the 3D7/Camp repeat precursors were lost in FC27 alleles, and the FC27 repeat precursors were lost in the 3D7 alleles.

As noted for Csp, the repetitive DNA sequences found within the

FIGURE 7. Nucleotide alignment of two Msp-2 gene sequences to manifest the repeats within RHR3 of P. falciparum OKS. This repeat region is not present in P. falciparum 3D7 or P. reichenowi. The repeat region of OKS continues contiguously from first to second to third to fourth row, left to right.

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