In Saccharomyces cerevisiae, G1/S transcription factors MBF and SBF regulate a 
large family of genes important for entry to the cell cycle and DNA replication 
and repair. Their regulation is crucial for cell viability, and it is conserved 
throughout evolution. MBF and SBF consist of a common component, Swi6, and a 
DNA-specific binding protein, Mbp1 and Swi4, respectively. Transcriptional 
repressors bind to and regulate the activity of both transcription factors. Whi5 
binds to SBF and represses its activity at the beginning of the G1 phase to 
prevent early activation. Nrm1 binds to MBF to repress transcription as cells 
progress through S phase. Here, we describe a protein motif, the GTB motif (for 
G1/S transcription factor binding), in Nrm1 and Whi5 that is required to bind to 
the transcription factors. We also identify a region of the carboxy terminus of 
Swi6 that is required for Nrm1 and Whi5 binding to their target transcription 
factors and show that mutation of this region overrides the repression of MBF- 
and SBF-regulated genes by Nrm1 and Whi5. Finally, we show that the GTB motif is 
the core of a functional module that is necessary and sufficient for targeting 
of the transcription factors by their cognate repressors.