IDENTIFICATION OF PROXIMAL SEQUENCE ELEMENT NUCLEOTIDES CONTRIBUTING TO THE DIFFERENTIAL EXPRESSION OF VARIANT-U4 SMALL NUCLEAR-RNA GENES

The two U4 genes in the chicken genome code for distinct sequence vari ants of U4 small nuclear RNA that are differentially expressed during development. Whereas U4B RNA is constitutively expressed, U4X RNA is s pecifically down regulated relative to U4B in a tissue-specific manner during development, To investigate mechanisms controlling the differe ntial expression of the U4B and U4X genes, chimeric U4 genes were cons tructed and their transcriptional activities assayed by injection into Xenopus oocytes or by transfection of CV-1 cells. The proximal regula tory region of the U4B gene and the enhancers of both the U4B and U4X genes functioned efficiently in each expression system, However, the p roximal region of the U4X gene was inactive. To localize and identify the responsible nucleotides, reciprocal point mutations were introduce d into the U4X and U4B proximal regulatory regions, The results indica te that the U4X gene contains a suboptimal proximal sequence element, and that this results primarily from the identities of the nucleotides at positions -61 and -57 relative to the transcription start site.