Despite 2 decades of research, the major function of the FUBP-family of KH domain proteins during animal development remains controversial. The literature is divided between RNA processing and transcriptional functions for these single stranded nucleic acid binding proteins. In Drosophila the three mammalian FUBP proteins are represented by one ortholog, Psi, and we have demonstrated the essential function of Psi in vivo is control of cell and tissue growth. Co-IP-mass spectrometry positioned Psi in an interactome predominantly comprised of RNA Polymerase II transcriptional machinery, including the transcriptional MEDIATOR (MED) complex, a known sensor of signaling inputs. Moreover, manipulation of MED activity modified Psi-dependent growth, which suggests Psi interacts with MED to integrate developmental growth signals. We demonstrate that the key target of the Psi/MED network in controlling tissue growth is the homolog of the MYC transcription factor and oncoprotein. As MYC dysregulation is associated with 70% of all cancers, this work will have implications for understanding the significance of FUBP and MEDIATOR complex dysfunction in human tumourigenesis.