SummaryTwo genes, EhgLE3 and Ehg34, encoding the ribosomal protein L21 (rp-L21) were identified and characterized from Entamoeba histolytica. Their coding regions are highly conserved, but their flanking regions differ significantly. Analogous genes (EdgLE3 and Edg34) were characterized in E. dispar. The two rp-L21 copies are transcribed at similar levels in the two parasites. However, their relative binding to the polyribosomal complex during active translation is different. In E. histolytica, binding of EhgLE3 transcripts to the polyribosomes is significantly higher in comparison with that of Ehg34 transcripts, whereas in E. dispar the binding pattern is inverse. The importance of each of the rp-L21 flanking regions to gene translation was investigated by constructing hybrid plasmids containing the CAT reporter gene flanked by rp-L21 flanking regions. The plasmids were stably transfected into E. histolytica and E. dispar, and CAT mRNA and enzymatic activity levels were determined. All plasmids promoted transcription of CAT. Yet, in E. histolytica, high levels of CAT activity were observed only when gLE3 upstream regions flanked CAT. In contrast, in E. dispar, high levels of CAT activity were observed when g34 upstream regions flanked CAT. The downstream regions showed no significant effect on CAT translation.

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