Analysis of the v-myb structural components important for transactivation of gene expression

In order to define the domains of the v-myb protein that are important for transactivation of gene expression, we have studied transactivation by the v-myb gene and a set of v-myb deletion mutants using transient transfection assays in NIH 3T3 cells. Analysis of the set of v-myb deletion products demonstrated that a previously unidentified region In the carboxyl-terminal portion of the protein is required for transactivation. This region lies between amino acids 295′356 with respect to the 5′end of the v-myb gene. Switching the v-myb DNA binding domain with the DNA binding domain of the rat glucocortlcold receptor (rGR) switched the cls-element requirement for v-myb action: only reporters containing glucocortlcold response elements were activated by myb-rGR fusion proteins. The carboxyl terminal region essential for transactivation by the intact v-myb gene was also necessary for transactivation by the rGR-fusion gene. Carboxyl-terminal deletion mutations that encompassed the novel transactivation region were able to block wild-type v-myb transactivation when tested In transient co-expression assays. In an unexpected sidelight to our studies, we could demonstrate that the lacZ gene present in the prokaryotic vector sequences contained a DNA element that fortuitously can act as a v-myb-dependent enhancer element, and that v-myb protein can bind to this element In vitro. The lacZ enhancer contains the myb consensus DNA binding site YAAC(G/T)G.