Dr. Phonphadit Xangsayalath �iNIOPH, Lao PDR/Nagasaki University 
Institute of Tropical Medicine�j 

abstract : 
Plasmodium falciparum SURFIN4.1 is a type I transmembrane protein 
localized on the surface of the merozoite. Analysis of the DNA sequence 
of the gene that encodes this protein in three laboratory-line parasites 
suggested that the N-terminal extracellular region was more polymorphic 
than the intracellular region. In order to investigate the level of 
polymorphism of the extracellular region of SURFIN4.1, and to assess 
whether this region is under positive selection,I evaluated the 2304 bp 
nucleotide (nt) sequence from 20 P. falciparum isolates collected in 
Thailand in 1988 and 1989. A total of 358 polymorphic sites were 
observed with an average nucleotide diversity of 0.064. Based on the 
predicted protein structure and the level of polymorphism, I divided the 
extracelluar region into three parts; a Cysteine-rich domain (nt 1-558), 
variable region 1 (nt 559-1512) and variable region 2 (nt 1513-2310). 
The majority of polymorphic sites were clustered in variable region 2. A 
significant excess of nonsynonymous substitutions over synonymous 
substitutions was detected in variable regions 1 and 2, suggesting that 
positive selection is acting on the variable region. Tajima�fs D test 
and Fu & Li�fs D* and F* tests all detected the signature of positive 
selection on variable regions 1 and 2 at the 98% confidence level. Thus, 
the variable regions 1 and 2 of SURFIN4.1 appear to have evolved under 
positive diversifying selection, possibly mediated by host immune 
pressure.