Tissue-specific post-transcriptional regulation of c-myc expression in normal and H-2K/human c-myc transgenic mice

We show that the steady-state levels of c-myc mRNAs vary considerably in different organs of normal adult mice, maximal expression being observed in lymphoid organs and minimal expression in liver and brain. Nuclear run-on analysis of c-myc gene transcription in adult liver and spleen reveals that the difference in c-myc gene expression in these two organs is due to differential post-transcriptional control. Moreover, these nuclear run-on assays indicate that no premature termination of c-myc gene transcription takes place in the nuclei of the three adult tissues analysed. In fetal liver development, we observe a decrease in c-myc mRNA, but this is not due to changes in transcriptional activity implicating post-transcriptional regulatory mechanisms. Our studies of c-myc gene expression in organs of H-2K/myc transgenic mice, harboring an H-2K promoter driven human c-myc gene, confirm that the in vivo c-myc regulation is mainly post-transcriptional and shows that sequences shared by the murine and human c-myc proto-oncogenes are involved in this control.