THE 5' AND 3' NONCODING SEQUENCES OF THE C-MYC GENE, REQUIRED INVITROFOR ITS POSTTRANSCRIPTIONAL REGULATION, ARE DISPENSABLE INVIVO

We have previously shown that in vivo the steady-state level of c-myc mRNA in different quiescent organs and its induction in the early stag es of hepatic regeneration and after inhibition of protein synthesis a re mainly controlled by post-transcriptional mechanisms. In order to l ocalize the target sequences for these mechanisms, transgenic lines ex pressing various versions of the human c-myc proto-oncogene have been constructed. To avoid all possible transcriptional controls due to the c-mye 5' regulatory region, the c-mye genomic sequences were fused to MHC H-2K(b) class I regulatory sequences, which have previously been shown to be able to drive reporter gene expression in most adult tissu es. The transgenes contained either all human c-mye genomic sequences or were deleted for one of the sequences which have been shown in in v itro experiments to play a role in c-mye mRNA stabilization, in partic ular exon 1, intron 1 and the 3' non-coding region. Several independen t transgenic lines were derived for each construct. Using S1 nuclease protection analysis, we have monitored H-2K, mouse c-myc and transgene mRNA expression in several quiescent adult organs, at the start of li ver regeneration and after inhibition of protein synthesis in each tra nsgenic line. Our results indicate that the 5' non-coding sequences, i ncluding exon 1 and intron 1, and the 3' untranslated region are all d ispensable in the different aspects of c-myc post-transcriptional regu lation.