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The Entamoeba histolytica rDNA episome: nuclear localization, DNAase I sensitivity map, and specific DNA‐protein interactions

Structural and functional features of the extrachromosomal DNA element that contains the ribosomal RNA genes of Entamoeba histolytica were studied using a variety of techniques. Using in situ hybridization, the element was found to be distributed along the inner phase of the nuclear membrane in the trophozoite stage; it appears to be part of the so‐called peripheral chromatin. DNAase I‐sensitive regions on the episome were mapped and found to correspond to the borders of the ribosomal RNA coding region. Other DNAase I‐sensitive regions were found to correspond to DNA containing a 145bp sequence that exists in the episome as tandem repeats. Electrophoretic shift assays and footprinting experiments demonstrate the existence of specific nuclear factors that bind specifically to the 145bp repeat. Preliminary analysis of the binding factors showed that a 28 kDa polypeptide is a likely candidate for a specific DNA:protein interaction involving the repetitive element. These results suggest that a protein‐binding domain within the 145bp repeat may have a specific function in the episome.