The aim of this proposal is to locate the regulatory elements required for transcription of rRNA genes in Entamoeba histolytica. Transcription of rDNA in E. histolytica is expected to be novel due to- A) the unusual arrangement of rRNA genes in this organism. These genes are exclusively episomal with no evidence of a chromosomal copy. It is known that in yeast, under conditions where the rRNA genes are episomal, these are transcribed by RNA polymerase II in place of Pol I and cryptic Pol II promoters of rRNA genes have been demonstrated (Conrad-Webb and Butow, 1995). B) The rDNA episome in some E. histolytica strains contains two rDNA units arranged as inverted repeats. The sequences upstream of the two rDNA units which normally contain the transcriptional regulatory elements are completely different from each other. This is a novel feature since in most organisms the multiple copies of rRNA genes have exactly identical upstream and downstream intergenic spacers. A notable exception is the protozoan parasite Plasmodium falciparum which contains two distinct classes of rRNA genes that are differentially expressed at different stages of the life cycle (Waters et at., 1989). In E. histolytica since the two rDNA units do not have identical upstream sequences it would be interesting to see if the two units are differentially expressed. Once the rDNA promoter and enhancer elements are identified these may be utilized for over expression of transfected genes in E. histolytica if the promoter strength is superior to the promoters currently used in E. histolytica vectors. In addition it is envisaged that knowledge about the protein factors that regulate rDNA transcription may provide novel drug targets to interfere with this essential process.