Transcriptional trans-repression by the c-myb proto-oncogene product.

We report that the c-myb protein binds to another site, MBS-II, in the SV40 enhancer with low affinity. In co-transfection experiments with a c-myb expression plasmid, tandem repeats of the sequence containing the MBS-II site induced c-myb-dependent transcriptional repression. Results of mutational analyses of the sequence around the MBS-II site suggested that the c-myb protein represses transcription by competing with another trans-activator. These results indicate that c-myb protein can regulate transcription not only positively but also negatively.