Expression of a mouse U1b gene in mouse L cells.

A 6.9-kb DNA fragment containing two mouse U1b genes was introduced by cotransfection with the Herpes simplex virus (HSV) thymidine kinase (TK) gene into tk- mouse L cells. The parent Ltk- cells produce primarily U1a RNA and only small amounts of U1b RNA. However, after introduction of exogenous U1b genes by cotransfection these cells express large amounts of U1b RNA. Subcloned DNA fragments containing a single U1b gene and varying amounts of DNA flanking the 5' -end of the gene were introduced into Ltk- cells. All of the constructs containing 400 bp or more upstream of the U1b gene were efficiently expressed. By comparison, a subclone containing only 150 bp of DNA flanking the 5' -end of the U1b gene was not efficiently expressed. The U1b transcripts synthesized in transfected cells were identified by hybrid selection and by precipitation of ribonucleoproteins (RNP) containing U1 RNA with anti-RNP antibody.