Xenopus laevis c-myc I and II genes: molecular structure and developmental expression.

The structure of the two Xenopus laevis c-myc I and c-myc II genes has been investigated by isolating and sequencing genomic and cDNAs clones. In oocytes, c-myc I mRNAs represent 80-90% of the overall amount of c-myc transcripts. The c-myc I expression is controlled primarily by two differentially regulated tandem promoters P1 and P2 which are separated by 50 bases. During oogenesis, maternal c-myc I mRNAs, are transcribed from both promoters whereas zygotic transcripts seem to initiate only from the P2 promoter. Sequence comparison between the promoter regions of c-myc I and II genes reveals the insertion in the c-myc I promoter region, between positions -831 and -389 relative to the P1 start site of a repetitive element. Comparison of X.laevis and mammalian c-myc promoter sequences reveals furthermore the conservation of cis-regulatory elements, including a motif known to be a negative regulator of the human c-myc transcription, a purine rich region, a binding site for the E2-F transcription factor and three SP1 binding sites. Finally, we report characterization of a new c-myc I mRNA which differ at the 5' end. Transcripts are possibly initiated at a putative alternative promoter located further upstream in the genome, and undergoes alternative splicing.