Increased transcription of the E mu-myc transgene and mRNA stabilisation produce only a modest elevation in Myc protein.

Mice bearing the E mu-myc transgene, which links the immunoglobulin heavy chain enhancer (E mu) with c-myc, are predisposed to developing B cell lymphomas. Several B lineage cell lines have been isolated from these animals, and some have been converted to macrophages following infection with v-raf. In this study we compared the regulation of myc expression in E mu-myc B lymphoma lines, their macrophage counterparts and other non-myc transformed B cell lines. Nuclear run-on analyses demonstrated that transcription of the transgene was elevated in E mu-myc B cell lines. Moreover, the presence of a 600 bp phiX174 marker in the 3' end of the transgene produced a marked stabilisation of this RNA species. Consequently, steady state myc mRNA levels in the E mu-myc B lymphoma cells were tenfold higher than the macrophage derivatives and non-myc transformed B lineage lines. Despite the considerable difference in myc RNA levels, the E mu-myc B cell lines contained only 30-50% more Myc protein than the other cell lines. This discrepancy between RNA and protein content was not due to increased degradation of the protein as the half life was normal in the transgenic cell lines. These results indicate that both E mu and phiX174 sequences influence transgenic myc expression and that protein levels do not correlate with RNA content in E mu-myc cell lines.