Sp-1 binds promoter elements that are regulated by retinoblastoma and regulate CTP:: Phosphocholine cytidylyltransferase-α transcription

The retinoblastoma (Rb) protein is implicated in transcriptional regulation of at least five cellular genes. Co-transfection of Rb and truncated promoter constructs has defined a discrete element (retinoblastoma control element (RCE)) within the promoters of each of these genes as being necessary for Rb-mediated transcriptional control. In the present report we demonstrate that two RCEs identified within the CTP: phosphocholine cytidylyltransferase-alpha (CT alpha) proximal promoter are essential to promote transcription. Mutations that abolished each RCE markedly decreased CT alpha transcription. Co-transfection of Rb and truncated promoter constructs demonstrated that Rb regulates CT alpha expression by different mechanisms depending on the phase of the cell cycle. The regulation of CT alpha expression by Rb required both the Sp1 and the RCEs. Maximal expression occurred when both Rb and Sp1 were overexpressed. Moreover, RCEs were required for Rb association with the DNA. This regulatory mechanism alters CT alpha activity and thereafter changes PC availability and cell physiology. This is the first report demonstrating not only that surrounding Sp1 binding sites alter regulation mediated by Rb, but also that the expression of a gene involved in PC biosynthesis shares a common regulatory pathway with genes responsible for cell growth and differentiation.