The C-terminal domain of B-myb acts as a positive regulator of transcription and modulates its biological functions

The myb gene family consists of three members, named A-, B-, and c-myb. All three members of this family encode nuclear proteins that bind DNA in a sequence-specific manner and function as regulators of transcription. In this report, we have examined the biochemical and biological activities of murine B-myb and compared these properties with those of murine c-myb. In transient transactivation assays, murine B-myb exhibited transactivation potential comparable to that of c-myb. An analysis of deletion mutants of B-myb and c- myb showed that while the C-terminal domain of c-Myb acts as a negative regulator of transcriptional transactivation, the C-terminal domain of B-Myb functions as a positive enhancer of transactivation. To compare the biological activities of c-myb and B-myb, the two genes were overexpressed in 32Dcl3 cells, which are known to undergo terminal differentiation into granulocytes in the presence of granulocyte colony stimulating factor (G- CSF). We observed that c-myb blocked the G-CSF-induced terminal differentiation of 32Dc13 cells, resulting in their continued proliferation in the presence of G-CSF. In contrast, ectopic overexpression of B-myb blocked the ability of 32D cells to proliferate in the presence of G-CSF and accelerated the G-CSF-induced granulocytic differentiation of these cells. Similar studies with B-myb-c-myb chimeras showed that only chimeras that contained the C-terminal domain of B-Myb were able to accelerate the G-CSF- induced terminal differentiation of 32Dc13 cells. These studies show that c- myb and B-myb do not exhibit identical biological activities and that the carboxyl-terminal regulatory domain of B-Myb plays a critical role in its biological function.