MOLECULAR-CLONING AND DEVELOPMENTAL EXPRESSION OF A SMALL RIBONUCLEARPROTEIN IN THE MOUSE TESTIS

U1 RNP C polypeptide is a ubiquitous and highly conserved protein that is found associated to the U1 small nuclear ribonuclear particle (U1 snRNP). The U1 snRNP is involved in pre-mRNA splicing by defining intr ons and exons and by binding to consensus sequences within the pre-mRN A. In the present study we immunoscreened a mouse testicular phagemid cDNA library with an anti-Sm serum from patients with systemic lupus e rythematosus. Sequence analysis of a positive clone containing a 0.75 kb cDNA insert revealed that it encodes the entire amino acid sequence of the U1 RNP C polypeptide. Northern blots of total RNA isolated fro m testes and various adult mouse tissues demonstrated that the 0.75 kb transcript is highly expressed in the testes and that it begins devel opmentally at day 18 postpartum, corresponding to the appearance of pr eleptotene spermatocytes. In situ hybridization confirmed the meiotic and postmeiotic expression of this transcript. LM immunoperoxidase sta ining with the anti-Sm serum localized spliceosome snRNPs predominantl y in the nuclei of somatic and germinal testicular cells but not in el ongated spermatids. EM immunogold labeling confirmed the LM observatio ns but additionally showed that snRNP content peaked in the nuclei of pachytene spermatocytes and that 2 cytoplasmic components found exclus ively in meiotic and post-meiotic germ cells were intensively reactive . Immunoblots of testicular homogenates probed with the anti-Sm serum revealed several reactive proteins, of which one, a 21 kDa polypeptide , could be the U1 RNP C based on its predicted molecular weight. In su mmary we report an isoform of U1 RNP C which is testis specific and wh ich may play a role in mRNA splicing exclusively in meiotic and post-m eiotic germ cells during spermatogenesis. (C) 1997 Wiley-Liss, Inc.