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Mol. Cell. Biol., Jan 1998, 499-511, Vol 18, No. 1
Copyright © 1998, American Society for Microbiology

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

IH Oh and EP Reddy
Fels Institute for Cancer Research and Molecular Biology, Philadelphia, Pennsylvania 19140, USA.

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 32Dcl3 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 32Dcl3 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.

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