Entry into the cell cycle in budding yeast involves transcriptional activation of G1cyclin genes and DNA synthesis genes when cells reach a critical size in late G1. Expression of G1cyclins CLN1 and CLN2 is regulated by the transcription factor SBF (composed of Swi4p and Swi6p) and depends on the cyclin-dependent Cdc28 protein kinase and cyclin Cln3p. To identify novel regulators of SBF-dependent gene expression we screened for mutants that fail to activate transcription of G1cyclins. We found mutations in a gene called CTR9. ctr9 mutants are inviable at 37 degrees C and accumulate large cells. CTR9 is identical to CDP1. CTR9 encodes a conserved nuclear protein of 125 kDa containing several TPR repeats implicated in protein-protein interactions. We show that Ctr9p is a component of a high molecular weight protein complex. Using immuno-affinity chromatography we found that Ctr9p associates with polypeptides of 50 and 65 kDa. By mass spectrometry these were identified as Cdc73p and Paf1p. We show that Paf1p, like Ctr9p, is required for efficient CLN2 transcription, whereas Cdc73p is not. Paf1p and Cdc73p were previously reported to be RNA poly-merase II-associated proteins, suggesting that the Ctr9p complex may interact with the general transcription apparatus.