Relative Increase in Polysomal mRNA for R1 cAMP‐Binding Protein in Neuroblastoma Cells Treated with 1,N6‐Dibutyryl‐adenosine 3′,‐5′‐phosphate

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Abstract
Polysomal RNA were isolated from control [mouse] neuroblastoma cells and those treated with 1,N6-dibutyryl-AMP (Bt2cAMP) and translated in wheat germ lysates. There was a specific induction in the synthesis of a protein, MW 48,000, by the polysomal RNA from Bt2cAMP-treated cells. This protein was identified as the R1 cAMP-binding protein by its coelectrophoresis with unlabeled binding protein and by its specific retention on 8-(6-aminohexylamino)AMP linked to sepharose. There was a 1.4 to 1.7-fold increase in the synthesis of the R1 cAMP-binding protein by polysomal RNAs isolated from Bt2cAMP-treated cells. There was a similar increase when purified polyadenylated mRNA populations were compared, showing there was no change in the ratio of adenylated to nonadenylated mRNA in the induced mRNA population. There was no corresponding increase in the synthesis of the R2 cAMP-binding protein although the relative synthesis of several other proteins was increased and the synthesis of actin and the .alpha. and .beta.-tubulin subunits was decreased. The increased levels of the R1 cAMP-binding protein found in Bt2cAMP-treated neuroblastoma cells are therefore partly caused by a specific accumulation of its mRNA on polysomes. The mRNA content of the cytoplasmic messenger ribonucleoprotein (mRNP) population of control cells was insufficient to account for this increase by a translocation of R1 mRNA from the mRNP to the polysome fraction in Bt2cAMP-treated cells. The increase in polysomal R1 mRNA is therefore caused by its increased transcription or post-transcriptional processing or its decreased rate of degradation in Bt2cAMP-treated cells. The specific induction of the mRNA for R1 cAMP-binding protein and the differential distribution of the R1 and R2 mRNA between the polysomal and messenger ribonucleoprotein compartments show that these 2 cAMP-binding proteins are encoded by different mRNA populations.