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Function of plant cell cycle genes

Gutierrez's group has focused his studies within the ECCO project on DEL cell cycle genes. E2F and DP form dimeric protein complexes with different functions in the cell cycle. One of the major functions of E2F/DP complexes described in animals is the control of G1/S transition. Plants also contain E2F and DP-like proteins. Based on their sequence, they cannot be unequivocally categorized as being either E2F or DP, since they share sequence similarity to both types of animal proteins. There is increasing evidence now for the existence of at least two different types of E2F. The first type is canonical E2Fs, which activate G1/S transition and DNA replication. E2Fa and E2Fb are of this type. The second group may actually function as repressors. They are also called DEL (DP-E2F-Like) proteins, because they are structurally quite divergent from the classical E2Fs. E2Fc is also an atypical E2F, as revealed by the studies of Gutierrez's group.

To investigate the role of some E2F/Dp proteins in different plant organs, Partner 3 has made transgenic plants in which the promoter sequences of E2F/Dp genes are fused to the GUS reporter gene to compare gene expression profiles. AtE2F2 (Acc.No.AF242581, E2Fc) is highly expressed in meristems and cotyledons in 1.5 day-old seedlings and in apical meristems and vascular tissues in 5 day-old seedlings. E2Fc is expressed in the basal part of developing leaves, roughly coincident with cell division activity in trichomes and in early stages of flower development.

At E2Ff (At3g01330; AtE2F4, AtELP1) is highly expressed in young cotyledons and leaves, hypocotyls and roots (but not in meristems), moderately expressed in flowers and not detectable in siliques (Ramirez-Parra et al., 2004). Arabidopsis E2Ff TDNA mutant (SALK Institute, USA) showed a strong growth reduction whereas 35S::E2Ff arabidopsis plants showed no phenotype. The authors concluded that growth is compatible with an altered expression of E2Ff. There was no clear function of E2Ff in cell proliferation. Rather it was concluded that E2Ff would act as a repressor in differentiated cells and could repress E2F target genes previously activated by another E2F from the activator class (E2Fa). Interestingly it was found that E2Ff binds to the promoter if several genes involved in cell wall modelling (Ramirez-Parra et al., 2004).

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