The variation of Pf155/RESA gene in field isolates of Plasmodium falciparum from Thailand.

Pf155/RESA, an antigen found on the surface of Plasmodium falciparum red blood cell membrane was once a proposed malarial vaccine candidate. The complete sequence of Pf155/RESA gene from one strain and partial sequence from two other isolates revealed that the gene is well conserved. But polymorphism of other antigenic encoded regions occurs with high frequency among isolates especially those collected from the field. Using solid phase sequencing technique, the nested PCR products of upstream 3' repeated region of exon 2 RESA gene were studied in 150 P. falciparum isolates. Of which 117 isolates were directly collected from the field and sequenced. Other samples studied include clones and cryopreserved of previously cultured isolates. The resulting sequences are compared with previously existing data of F32 (Tanzania) and FC27 (Papua New Guinea) designated as allelic type I and II respectively. Sequence analysis of the 150 P. falciparum showed that the amplified region of RESA gene was highly variable with substitution ranging from one to six bases and these allelic variables can be divided into 10 types. The frequency of type I(F32) occurrence is 70.86 percent, type III 13.38 per cent and 0.78 percent to 5.51 per cent for others. As a result of allelic polymorphism, the amino acid sequence is highly variable and this may cause Pf155/RESA to be an inefficient antigen.