Transcriptional Regulation During the Plant Cell Cycle: Involvement of Myb Proteins in Cyclin B Transcription
The eukaryotic cell cycle is controlled so that the temporal order of DNA replication, nuclear division, and cytokinesis takes place in a well-defined and reproducible order. The periodic activation of the genes at the transcriptional level plays a role for these controls. Such periodically expressed genes frequently encode proteins that either control cell cycle progression directly or function in essential cellular processes linked to the cell cycle (Koch and Nasmyth 1994; Müller 1995). During the G1-S transition, a set of specific genes is induced, including G1 cyclin genes and several other genes involved in DNA synthesis. G1/S-specific transcription is regulated by mechanisms involving the E2F/DP heterodimeric transcription factors in animal cells (Black and Azizkhan-Clifford 1999), and by MBF (MluI cell cycle box-binding factor) and SBF (Swi4/Swi6 cell cycle box-binding factor) in yeast (Saccharomyces cerevisiae; Koch and Nasmyth 1994). Plants have genes encoding for E2F and DP factors similar to animal counterparts, but do not have genes for factors similar to the components of yeast SBF and MBF. The tobacco E2F gene, NtE2F, is expressed in late G1 and S phases in synchronized tobacco BY-2 cells, suggesting a role in G1/S-specific transcriptional regulation (Sekine et al. 1999). Arabidopsis E2F, in association with DP, can bind to DNA sequences similar to those of animal E2F binding sites, and transactivate E2F-responsive promoters when transfected into plant cells (Kosugi and Ohashi 2002). Furthermore, several S phase-specific genes are up-regulated in transgenic Arabidopsis plants that overexpress both E2F and DP simultaneously (De Veylder et al. 2002). These findings support the idea that plants have evolved a mechanism for G1/S-specific transcription that relies on genes analogous to those acting in animal cells. In contrast, recent studies have shown that plants employ a unique mechanism for G2/M phase-specific transcription. In plants, a group of Myb transcription factors bind to a cis element that is sufficient for G2/M phase-specific promoter activation. These factors act for transcriptional activation and repression of a set of G2/M phase-specific genes, including Btype cyclin genes. In animals, it has been proposed that a group of G2/M phase-specific genes is regulated by several different cis -acting elements, but no single cis element has been shown to be sufficient for G2/M phase-specific transcription. It seems likely that periodic transcription at the G2/M phase in animal cells is achieved by a combination of different cis elements, which act as repressors and activators.
KeywordsCyclin Gene Plant Cell Cycle Human Cyclin Tandem Element
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