Abstract
Cysteine proteinases of Entamoeba histolytica are considered to be one of the most important classes of molecules responsible for the parasite's ability to destroy human tissues. Interestingly, one particular cysteine proteinase, located on the surface of E. histolytica trophozoites and designated cysteine proteinase 5 (CP5), is not expressed in the closely related but nonpathogenic species Entamoeba dispar. By comparing the E. histolytica and E. dispar genomic loci containing the gene for CP5 (cp5), it was found that the position of cp5 within the genomic context is conserved between the two organisms, but that the gene is highly degenerated in E. dispar, as it contains numerous nucleotide exchanges, insertions, and deletions, resulting in multiple stop codons within the cp5 reading frame. An alignment of all available orthologous E. histolytica and E. dispar DNA sequences suggested that cp5 started to degenerate in E. dispar coincidently when the two organisms began to diverge from a common ancestor.