Fig. 6. U1A-binding motifs inhibit accumulation of polyadenylated µ secretory mRNA in vivo. RNase protection assays. Wild-type or mutant plasmids containing the IgM heavy chain gene were transfected into M12.4.2 cells in triplicate. (A) RNase protection assay visualized by phosphorimagery. Lane 1 is the ³²P-radiolabeled pBR322 MspI digest marker. Lanes 2–4 are mock, wild-type and mutant transfected poly(A)-selected mRNA, respectively. The secretory and membrane-protected fragments are indicated with brackets. (B) Phosphorimagery analysis of (A). The secretory measurements were corrected for the higher level of [³²P]UTP due to a longer protected fragment and all the results were expressed as a ratio of the wild-type membrane-protected fragment. Bars represent the mean of triplicates ± SE.