Distinct Cellular Factors Regulate the c- myb Promoter through Its E2F Element

Abstract

ABSTRACT Most E2F-driven promoters are transiently activated around the G 1 /S transition. Although the promoter for the c- myb proto-oncogene harbors an E2F element, it is induced early in G 1 following entry into the cell cycle. Furthermore, this promoter remains active throughout subsequent cell cycles. Since E2F sites function as repressor elements during G 1 (due to the association of pRb with E2F factors), we investigated whether the E2F element in the c- myb promoter is regulated differently than E2F elements in promoters that are repressed during G 1 . By gel shift analysis, the E2F element from the c- myb promoter was found to form a unique complex, referred to as E2Fmyb-sp, which was not observed with E2F elements from several other promoters. Antibodies to DP-1, E2F1 to -5, p107, or pRb failed to either supershift or block E2Fmyb-sp complex formation. Methylation interference experiments indicate that the DNA contact residues for the E2Fmyb-sp complex are distinct from but overlapping with residues required for the binding of E2F proteins. In addition to the identification of E2Fmyb-sp, we have found that SP-1 binds to the c- myb E2F element. Functional studies revealed that E2Fmyb-sp and/or SP-1 are required to achieve full activation of the c- myb promoter in different cell types and to maintain elevated expression of the c- myb promoter during G 1 in NIH 3T3 cells. These studies demonstrate that E2F elements can be regulated differently through the binding of unique sets of proteins.