Transcription regulation by murine B-mybis distinct from that by c-myb

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Abstract
The transcription regulatory properties of murine B-myb protein were compared to those of c-myb. Whereas c-Myb trans-activated an SV40 early promoter containing multiple copies of an upstream c-Myb DNA blndlng site (MBS-1), and similarly the human c-myc promoter, B-Myb was unable to do so. Full-length B-Myb translated in vitro did not bind MBS-1; however, truncation of the B-Myb C-terminus or fusion of the B-Myb DNA-binding domain to the c-Myb C-terminus showed that it was Inherently competent to Interact with this motif. Further evidence from co-transfectlon experiments, demonstrating that B-Myb Inhibited trans-activation by c-Myb, suggested that failure of B-Myb to trans-activate these promoters did not simply occur through lack of binding to MBS-1. Moreover, using GAL4B-Myb fusions, it was found that an acidic region of B-Myb, which by comparison to c-Myb was expected to contain a transcription activation domain, actually had no Inherent trans-activation activity and indeed appeared to trans-Inhibit c-Myb. In contrast to the above findings, both B-Myb and c-Myb were able to weakly trans-activate the DNA polymerase alpha promoter. Results obtained here demonstrate that the activities of B-Myb and c-Myb are clearly distinct and suggest that these related proteins may have different functions In regulation of target gene expression.