Expression of histone-U1 snRNA chimeric genes: U1 promoters are compatible with histone 3' end formation

Chimeric genes which fuse the mouse histone H2a gene and the mouse U1b gene were constructed and introduced into CHO cells by cotransfection. In the UH genes, the U1b gene promoter and the start of the U1b gene were fused to the H2a gene in the 5' untranslated region. In the HU genes, the U1b 3' end was inserted into the 3' untranslated region of the H2a gene replacing the normal histone 3' end. Transcripts from the UH genes initiated at the start of the U1 gene and ended at the normal histone 3' end. Transcripts from the HU chimeric genes did not end at the U1 3' end but extended at least 80 nucleotides further and had heterogeneous 3' ends. Placing both a U1 snRNA promoter and a U1 snRNA 3' end around a histone coding region resulted in transcripts which initiate and terminate at the appropriate U1 ends. These results are consistent with previous reports that formation of the U1 3' ends require U1 promoters, but indicate that the histone 3' end can be formed on transcripts initiating at U1 promoters. The transcripts initiated at the U1 start site and ending at the histone 3' end are present on polyribosomes and show proper posttranscriptional regulation.