The 5S ribosomal RNA gene of Euplotes

A library of Euplotes eurystomus macronuclear DNA was constructed in the pUC plasmid vector and cloned in E. coli. Screening of the library with the 5S ribosomal RNA gene of Tetrahymena thermophila resulted in 19 apparently identical isolates of the Euplotes 5S gene. The gene was subcloned into bacteriophage Ml 3 and its sequence was determined by the Sanger method. The Euplotes macronuclear 5S rRNA gene is 930 bp in length, is flanked by telomeres, and contains a single 5S rRNA coding region. This gene serves as the template for a single transcript of 120 nucleotides in vivo and in vitro; this transcript is generated by RNA polymerase III. The 5S gene is amplified in the Euplotes macronucleus approximately 120-fold compared to a gene of typical abundance, amounting to about 960,000 copies per macronuclear genome. The Euplotes macronuclear 5S rRNA gene can be enriched as a native, intact chromatin structure by sucrose gradient centrifugation; in the peak fraction about 1 in 30 chromatin molecules harbors a 5S gene.