Vol. 164, No. 1, 1989 October 16, 1989 BIOCHEMICAL AND EIOPHYSICAL RESEARCH COMMUNICATIONS Pages 176-184 Isolation and Characterization of a Molecular cDNA Clone of a Human mBNA from Interferon-treated Cells Encoding Nucleolar Protein B23, Numatrin"~' Xuxian T. Zhang, Daniel C. Thomis and Charles E. Samuel Section of Biochemistry and Molecular Biology Department of Biological Sciences University of California, Santa Barbara, CA 93106 Received August 21, 1989 SUMMARY : A cDNA clone encoding human nucleolar phosphoprotein B23, numatrin, was isolated from a library prepared with mPNA from human U cells. The complete nucleotide sequence was determined: it revealed a single open reading frame of 294 amino acids that included four in-frame AUG codons as potential sites of translation initiation. Comparison of the human B23 nucleotide sequence with the rat and mouse B23 sequences revealed 91% homology (hum:rat, and hum:mus) in the coding region: the predicted B23 proteins displayed 94% amino acid identity. Northern gel blot analysis revealed a single B23 mRNA species of -1.5 kb. The level of B23 mRNA in U cells was not detectably altered by treatment with either alpha or gamma interferon. Southern gel blot analysis revealed polymorphism within the human B23 gene structure, and suggested the presence of multiple B23 genes and/or extensive splicing of B23 RNA transcripts. 01989Acade~ic Press, Inc. Protein B23 i.5 a major phosphoprotein component of the granular region of the nucleolus (l-4), the subcellular site of assembly of pre-ribosomal RNAs with proteins (5). Although the biochemical function of protein B23 has not yet been precisely established, the observations that protein B23 is found in the nucleolus (l-4), binds to single-stranded nucleic acids including WA, and is associated with pre-ribosomal ribonucleoprotein *This work wes supported in pert by Research Grants AI-12520 end ~1-20611 from the National Institute of Allergy end Infectious Diseases, IJ. S. P. H. S. 1 The nucteotide sequence reported in this paper has been submitted to GenBank, Los Alamos, New Mexico, with eccession number W26697. The abbreviations used ere: IFN, interferon; 823, protein 823, numatrin; nt. nucleotide; ORF, open reading frame. 0006-291X/89 $1.50 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved. 176