Activation of cryptic promoters of human c-myc genes in microinjected Xenopus laevis oocytes.

The human cellular oncogene c-myc contains two promoters at the 5' end of its first non-coding exon. Cryptic promoters located within the first intron are also activated to synthesize aberrant c-myc mRNAs in some Burkitt lymphomas with a t(8: 14) chromosome translocation in which a part of the gene structure, often the 5' non-coding exon, is truncated. We have shown elsewhere that microinjected plasmid DNA carrying a normal, intact human c-myc gene directs efficient faithful transcription from its own two promoters in Xenopus laevis oocytes. Here, I have investigated the expression of different recombinants carrying various constructs of the c-myc gene in frog oocytes in order to understand the activation mechanism of those cryptic promoters. Aberrant c-myc transcripts initiating from cryptic promoters within the first intron are undetectable when the intact or truncated c-myc gene construct is used. However, the cryptic promoters can be activated in Xenopus oocytes if the truncated c-myc gene construct is fused with simian virus 40 sequences containing a 21 base-pair repeat and the replication origin. Xenopus oocytes will be useful for further investigation of enhancing elements involved in the translocated and activated c-myc genes in Burkitt lymphoma cells.