Mediator is a transcription co-factor complex that co-operates with transcriptonal activators to enhance gene specifc transcription and is conserved between yeast Drosophila and man. The MED17 subunit of Mediator (formerly known as TRAP80/CRSP6/DRIP80) has been characterised as a transcriptional activator interacting with a number of transcription factors, such as heat shock factor and p53. Expression of MED17 in yeast and Drosophila is essential to cell viability possibly due to its function as a global transcriptional regulator. In a yeast-2-hybrid screen with a viral cyclin as bait, MED17 was identified as an interacting clone. Due to the oncogenic potential of viral cydins, effects of human MED17 on p53 regulated transcription were investigated. Functional characterisation of MED17 effects on p53 showed that it repressed p53 mediated transcription in lucrferase reporter assays. Further, a MED17 constitutively expressing line generated in non-transformed mouse cells inhibited apoptosis and demonstrated other features of p53 functional loss. Human MED17 still activates heat shock regulated transcription, as previously described for the Drosophila homologue. Analysis of other transcription factors regulated by MED17 was investigated by gene expression microarray analysis of the MED17 cell line, revealing a putative co-activator function in B-eatenin regulated transcription. Also studied was the interaction of MED17 with cellular homologues of viral cyclin. Cyclin/cdks phosphorylate MED17, with cyclin A/cdk2 specifically phosphorylating MED17 to enhance its expression. This investigation reveals a novel repressor function for MED17 on p53 mediated transcription and links cell cycle regulators to the transcriptional activities of MED17/Mediator and p53.