SEQUENCE ORGANIZATION OF THE ACANTHAMOEBA RIBOSOMAL-RNA INTERGENIC SPACER - IDENTIFICATION OF TRANSCRIPTIONAL ENHANCERS

The primary sequence of the entire 2330 bp intergenic spacer of the A. castellanii ribosomal RNA gene was determined. Repeated sequence eleme nts averaging 140 bp were identified and found to bind a protein requi red for optimum initiation at the core promoter. These repeated elemen ts were shown to stimulate rRNA transcription by RNA polymerase I in v itro. The repeats inhibited transcription when placed in trans, and st imulated transcription when in cis, in either orientation, but only wh en upstream of the core promoter. Thus, these repeated elements have c haracteristics similar to polymerase I enhancers found in higher eukar yotes. The number of rRNA repeats in Acanthamoeba cells was determined to be 24 per haploid genome, the lowest number so far identified in a ny eukaryote. However, because Acanthamoeba is polyploid, each cell co ntains approximately 600 rRNA genes.