Chromatin control during the cell cycle

Maintaining cells in the quiescent G0 phase of the cell division cycle is achieved by suppressing the expression of genes required for cell-cycle progression. In the May 10 Science, Hidesato Ogawa and colleagues at the Dana-Farber Cancer Institute in Boston describe a repressive mechanism in quiescent cells involving the E2F-6 transcription factor (Science 2002, 296:1132-1136). They immunopurified protein complexes that include E2F-6 from human cells and analysed co-purifying protein bands by mass spectrometry. They detected the DP-1, Mga and Max transcription factors and showed that the E2F-6 complex can bind to E2F- and Myc-binding sites on DNA; they also found two associated histone methyltransferase enzymes. In addition, Ogawa et al.detected the HP1γ protein that binds to methylated lysine 9 residues in the tails of histone proteins. Finally, they also found evidence for Polycomb-group-like, ring-finger proteins in the same complex. The authors propose that E2F- and Myc-regulated genes are repressed in quiescent cells by E2F-6 and recruitment of a complex that modifies chromatin structure.