We report that the c-myb protein binds to another site, MBS-l1, in the SV4O enhancer with low affinity. In co-transfection experiments with a c-myb expression plasmid, tandem repeats of the sequence containing the MBS-I1 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 the c-myb protein can regulate transcription not only positively but also negatively.
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