Position and orientation independent transactivation by c-Myc.

The c-myc oncogene c-Myc is commonly activated in cancer and transactivates gene expression by binding to CACGTG DNA sequences as a heterodimeric complex with Max. The ornithine decarboxylase (ODC), p53, prothymosin alpha and ECA39 promoters are transactivated by c-Myc, and are considered direct targets, as activation is mediated by CACGTG sequences. Interestingly, the c-Myc-responsive CACGTG sequences in the p53, prothymosin alpha, ECA39 and murine ODC genes are all downstream of the RNA CAP site, suggesting that downstream sequences are preferred c-Myc targets. Using a series of heterologous reporter constructs, we have tested the effects of position and orientation of c-Myc-responsive CACGTG sequences on c-Myc's ability to activate transcription. A single binding site conferred c-Myc-responsiveness independent of position and orientation, and over distances of 1.7 kbp. The extent of transactivation was not significantly influenced by position of the responsive elements. By contrast, the extent of transactivation was dependent upon the number of c-Myc binding sites. The results demonstrate that c-Myc activates transcription independent of position and orientation and that considerable flexibility exists in the interaction of c-Myc transactivation domains with the general transcription machinery.