Activation of transcription by v-myb: evidence for two different mechanisms.
The retroviral oncogene v-myb encodes a nuclear, sequence-specific DNA-binding protein. To investigate the possibility that v-myb encodes a transcriptional regulator, we used a transient cotransfection assay to explore the potential of v-myb to influence the expression of other genes. We found that expression of a chicken lysozyme promoter/CAT gene construct was activated by v-myb in the presence of myb-specific binding sites. Action was not observed with a truncated v-myb protein lacking its DNA-binding domain. We also observed that expression of a hybrid human HSP70 promoter/CAT gene, lacking myb-specific binding sites, was activated by v-myb. However, in this case, the truncated v-myb protein, which lacked its DNA-binding domain, also activated HSP70/CAT expression, indicating that trans-activation of this gene construct was independent of the sequence-specific DNA-binding activity of the v-myb protein. These observations suggest that v-myb encodes a trans-activator and that activation of gene expression occurs by two different mechanisms, one of which involves specific binding of v-myb protein to the regulated gene.