A DNA sequence corresponding to the gene encoding cysteine proteinase 5 inEntamoeba histolytica is present and positionally conserved but highly degenerated in Entamoeba dispar

Cysteine proteinases of Entamoeba histolytica are considered to be one of t he most important classes of molecules responsible for the parasite's abili ty to destroy human tissues. Interestingly, one particular cysteine protein ase, located on the surface off. histolytica trophozoites and designated cy steine proteinase 5 (CP5), is not expressed in the closely related but nonp athogenic species Entamoeba dispar. By comparing the E. histolytica and E. dispar genomic loci containing the gene for CP5 (cp5), it was found that th e position of cp5 within the genomic context is conserved between the two o rganisms, but that the gene is highly degenerated in E. dispar, as it conta ins numerous nucleotide exchanges, insertions, and deletions, resulting in multiple stop codons within the cp5 reading frame. An alignment of all avai lable 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.