The 11-1 gene of Plasmodium falciparum codes for distinct fast evolving repeats

The 11-1 gene of Plasmodium falciparum has been investigated by DNA sequence analysis. It begins at the 5' end with a putative miniexon coding for a polypeptide which has the characteristics of a signal sequence. The miniexon is followed by a small intron. This again is followed by a large exon consisting of 9-, 18- and 27-bp repeats embedded in unique DNA. Specific antibodies isolated by affinity chromatography on a purified recombinant fusion protein expressing the three- and six-amino acid repeats were used to identify the product of the 11-1 gene. In exhibits size variations from 260 to 350 kd in different strains. Southern blot analysis with synthetic DNA as probe demonstrates that the 18-bp repeat is absent or drastically altered in two strains whereas the other repeats are present in all seven strains investigated. The unusual preference for G in the third position of some codons of the repeats but not in the unique sequences indicates rapid evolution of the repeats. Slippage during replication, unequal crossing over and selection are discussed as possible mechanisms leading rapidly to extreme diversity.