Mycophenolic acid (MPA) is the active metabolite of Mycophenolate Mofeteil (MMF), an effective immunosuppressive drug. Both MPA and MMF are highly specific inhibitors of guanine nucleotide synthesis and of T cell activation. However, the mechanism by which guanine nucleotide depletion suppresses T cell activation is unknown. Depletion of GTP inhibits ribosomal RNA synthesis in T cells by inhibiting TIF-IA, a GTP-binding protein that recruits RNA Polymerase I to the ribosomal DNA promoter. TIF-IA-GTP binds the ErbB3 binding protein 1 (Ebp1) and together they enhance the transcription of proliferating cell nuclear antigen (PCNA). GTP binding by TIF-IA and Ebp1 phosphorylation by protein kinase C delta are both required for optimal PCNA expression. The PKC inhibitor Sotrastaurin markedly potentiates the inhibition of rRNA synthesis, PCNA expression, and T cell activation induced by MPA, suggesting that the combination of the two agents are more highly effective than either alone in inducing immunosuppression.