Comparison of the mouse L32 ribosomal protein promoter elements in mouse myoblasts, fibers, and L cells

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Abstract
The sequences required for the maximal expression of the mouse L32 ribosomal protein gene and the binding of nuclear factors to L32 promoter elements were analyzed in mouse myoblasts, fibers, and L cells. Various L32 r-protein promoter sequences were linked to the chloramphenicol acetyltransferase gene (CAT), and the expression of the chimeric genes was measured transiently or after their incorporation into the genome. The sequence requirements for maximal expression of the L32 gene are very similar among the various cells and include the previously identified L32 core promoter from approximately -150 to +75. Only the promoter regions between -45 and +11 displays significant cell type specific differences. Relative to the maximal activity in each cell type, the expression of the L32-CAT gene containing the -45 to +11 region is greater in L cells than in myoblasts or fibers. This difference is correlated with the increased activity of an L cell nuclear factor(s) that binds to this fragment. In addition, our results show that deletion of sequences between -981 and -141 causes a 50-70% reduction of the expression of the L32-CAT gene in myoblasts, fibers and L cells. The transcription of all the L32-CAT genes examined decrease after myoblasts differentiate into fibers in a manner similar to the endogenous L32 gene, but we were unable to distinguish between sequences involved in controlling the expression of the L32 gene during myoblast differentiation and those sequences required for maximal promoter activity. However, gel mobility shift assays showed differences in the binding of myoblast and fiber factors to the four promoter fragments examined. The possible role of these factor binding differences in controlling L32 transcription is discussed.