Cell cycle regulation of a novel DNA binding complex in Saccharomyces cerevisiae with E2F-like properties

Using a biochemical approach, we have detected an activity in Saccharomyces cerevisiae extract that displays the same DNA binding specificity as the mammalian E2F transcription factor and interacts with TTTCGCGC promoter elements. Additional studies revealed that this factor, termed SCELA (S. cerevisiae E2F-like activity), also binds to the closely related SCB promoter sequences. SCB sites (consensus: TTTCGTG) are involved in the cell cycle regulation of several S. cerevisiae cyclin genes and have been shown to interact with the heterodimeric yeast Swi4-Swi6 complex. However, genetic studies clearly demonstrate that SCELA is not related to Swi4 or Swi6. These experiments imply that SCB sites are able to interact with at least two activities: Swi4-Swi6 and SCELA. Because SCB sites are critical for the periodic activation of cell cycle genes, we asked whether SCELA is regulated during yeast cell cycle. Employing a temperature-sensitive strain, we were able to demonstrate that the DNA binding activity of SCELA oscillates during the cell cycle and reaches its maximum at the transition between the G1 and S phases. Preliminary studies suggest that this fluctuation is mediated by phosphorylation/dephosphorylation events. Further characterization of SCELA by UV cross-linking experiments indicate a molecular mass of 47 kDa for this activity. In addition, we present evidence strongly suggesting that SCELA is actually the DNA binding moiety of a large 300-kDa protein complex. Together, these studies firmly indicate that SCELA (as part of a larger complex) plays a critical role in cell cycle regulation of SCB-containing genes, such as CLN cyclins and HO endonuclease. This hypothesis is consistent with other studies that conclude that the SCB-mediated cell cycle oscillation of CLN cyclins and HO requires activities that are distinct from Swi4-Swi6. Finally, it is worth mentioning that the similarities between SCELA and E2F, which is a crucial component in mammalian cell cycle regulation, extend well beyond the DNA binding specificity. In analogy to E2F, SCELA oscillates during the cell cycle, interacts with other cellular activities, and binds to promoter elements that are known mediators of cell cycle control. We will discuss possible functions for SCELA in yeast cell cycle regulation and its relationship to E2F.