Regulation of the cyclin D1 and cyclin A1 promoters by B-Myb is mediated by Sp1 binding sites.

B-Myb is a highly conserved member of the Myb family of transcription factors which plays an important role during the cell cycle. Previous work has shown that B-Myb is phosphorylated at several sites by cyclin A/Cdk2 in the early S-phase. These phosphorylations increase the transactivation potential of B-Myb by counteracting the repressive function of an inhibitory domain located at the carboxyl-terminus of B-Myb. As yet, only a few genes have been identified as B-Myb target genes. Previous work has suggested that the cyclin D1 gene might be regulated by B-Myb. Here, we have studied the effect of B-Myb on the promoter of the cyclin D1 gene. We show that B-Myb is a potent activator of the cyclin D1 promoter and that this activation is not mediated by Myb binding sites but rather by a group of Sp1 binding sites which have previously been shown to be crucial for cyclin D1 promoter activity. Our data show that the C-terminal domain of B-Myb is required for the activation of the cyclin D1 promoter and that this part of B-Myb interacts with Sp1. Finally, we have found that the promoter of the cyclin A1 gene is also activated by B-Myb by a Sp1 binding site-dependent mechanism. The effect of B-Myb on the promoters of the cyclin A1 and D1 genes is reminiscent of the mechanism that has been proposed for the autoregulation of the B-myb promoter by B-Myb, which also involves Sp1 binding sites. Taken together, our identification of two novel B-Myb responsive promoters whose activation by B-Myb does not involve Myb binding sites extends previous evidence for the existence of a distinct mechanism of transactivation by B-Myb which is dependent on Sp1 binding sites. The observation that this mechanism is not subject to the inhibitory effect of the C-terminal domain of B-Myb but rather requires this domain supports the notion that the Sp1 site-dependent mechanism is already active in the G1-phase prior to the phosphorylation of B-Myb by cyclin A/Cdk2.