Retrovirus integration site Mintb encoding the mouse homolog of hnRNP U

Retroviral genes are not usually expressed in mouse embryonal carcinoma (EC ) cells, but they are readily expressed upon differentiation of these cells . We previously reported the isolation of EC cell lines that express a neom ycin resistance (neo) gene introduced by a recombinant transducing Moloney murine leukemia virus from specific integration sites, Minta, Mintb, Mintc, or Mintd, In some of these clones, the entire 5' long terminal repeat (LTR ) was deleted, and the neo gene was expressed by read-through transcription from upstream cellular promoters in a "promoter-trap" fashion, One such pr omoter ("promoter B" at the Mintb locus) was found in a CPG island, associa ted with an upstream enhancer ("enhancer B"), Although enhancer B caused ex pression of the neo gene in the transductant EC cell line, no endogenous tr anscription from promoter B was detected in the parental EC or NIH3T3 cells . In contrast, we found a strong counter-flow endogenous transcription unit ("R" for reverse), which apparently interfered with transcription from pro moter B, Promoter R turned out to have a bidirectional activity in transfec tion assays. In normal tissues, promoter R activates gene R, which encodes an 800-residue protein that is highly homologous to the rat and human heter ogeneous nuclear ribonucleoprotein U (hnRNP U), Northern and in situ hybrid ization analyses revealed that gene R was abundantly expressed in the testi s, especially in the pachytene spermatocytes and round spermatids.