AN ABUNDANT NUCLEOLAR PHOSPHOPROTEIN IS ASSOCIATED WITH RIBOSOMAL DNAIN TETRAHYMENA-MACRONUCLEI

An abundant 52-kDa phosphoprotein was identified and characterized fro m macronuclei of the ciliated protozoan Tetrahymena thermophila. Immun oblot analyses combined with light and electron microscopic immunocyto chemistry demonstrate that this polypeptide, termed Nopp52, is enriche d in the nucleoli of transcriptionally active macronuclei and missing altogether from transcriptionally inert micronuclei. The cDNA sequence encoding Nopp52 predicts a polypeptide whose amino-terminal half cons ists of multiple acidic/serine-rich, regions alternating with basic/pr oline-rich regions. Multiple serines located in these acidic stretches lie within casein kinase II consensus motifs, and Nopp52 is an excell ent substrate for casein kinase II in vitro. The carboxyl-terminal hal f of Nopp52 contains two RNA recognition motifs and an extreme carboxy l-terminal domain rich in glycine, arginine, and phenylalanine, motifs common in many RNA processing proteins. A similar combination and ord er of motifs is found in vertebrate nucleolin and yeast NSR1, suggesti ng that Nopp52 is a member of a family of related nucleolar proteins. NSR1 and nucleolin have been implicated in transcriptional regulation of rDNA and rRNA processing. Consistent with a role in ribosomal gene metabolism, rDNA and Nopp52 colocalize in situ, as well as by cross-li nking and immunoprecipitation experiments, demonstrating an associatio n between Nopp52 and rDNA in vivo.