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.
Bartusel
Thorsten
T
Institut für Biochemie, Westfälische-Wilhelms-Universität Münster, Germany.
Schubert
Stephan
S
Klempnauer
Karl-Heinz
KH
D000818:Animals; D001665:Binding Sites / Q000235:genetics; D045744:Cell Line, Tumor; D006224:Cricetinae; D019925:Cyclin A / Q000235:genetics*; D056750:Cyclin A1; D019938:Cyclin D1 / Q000235:genetics*; D005822:Genetic Vectors / Q000235:genetics; D005982:Glutathione Transferase / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D051379:Mice; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics*; D011485:Protein Binding; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D016329:Sp1 Transcription Factor / Q000378:metabolism*; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D020798:Two-Hybrid System Techniques; D001616:beta-Galactosidase / Q000235:genetics / Q000378:metabolism
C431443:Bmyc protein, mouse; C503745:CCNA1 protein, human; C503747:Ccna1 protein, mouse; D019925:Cyclin A; D056750:Cyclin A1; D009687:Nuclear Proteins; D016271:Proto-Oncogene Proteins c-myc; D011993:Recombinant Fusion Proteins; D016329:Sp1 Transcription Factor; D015534:Trans-Activators; D019938:Cyclin D1; D008156:Luciferases; D005982:Glutathione Transferase; D001616:beta-Galactosidase