Interaction of the yeast Swi4 and Swi6 cell cycle regulatory proteins in vitro.

In budding yeast, two transcription factors, Swi4 and Swi6, control the expression of important cell cycle regulatory proteins (the G1 cyclins, Cln1 and Cln2, and the cyclin-like Hcs26) as well as the HO gene, whose product initiates mating-type switching. Both Swi4 and Swi6 are components of a protein complex that forms at a repeated sequence element, SCB (SWI4, -6-dependent cell cycle box), found in the upstream regulatory sequences of target genes. We show, by using proteins synthesized in vitro, a direct association between Swi4 and Swi6. The cdc10-Swi6 or ankyrin motifs present in both Swi4 and Swi6 are dispensable for their association, which is mediated instead by a region near the C terminus of each protein. Furthermore, we show that interaction with Swi6 is not necessary for specific recognition of the SCB sequence by the Swi4 protein; we propose that Swi4 is responsible for binding to the SCB sequence while Swi6, through its association with Swi4, regulates activity of the complex.