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Function of plant cell cycle genes

Bergounioux's group has studied the regulation of the plant pre-replication complex. This is important for understanding both G1/S transition as endoreduplication. The screen of the tagged mutant library of INRA did not give any knockout mutants for the genes of the pre-replication complex. Therefore, the strategy was changed to the construction and analysis of anti-sense, over-expressing and RNAi plants for cdc6-1, cdc6-2, cdc45, MCM2 and MCM3.

For cdc6-1 and cdc6-2 over-expression, second and third generation plants have been analysed. Cdc6-1 over-expression did not induce endoreplication suggesting that, opposite to S. pombe but, similarly to S. cerevisiae and other eukaryotes re-initiation of the replication should require simultaneous down-regulation of several pathways.

Cdc45 is required for the initiation of DNA replication in yeast, cell proliferation in mammals and functions as a DNA polymerase alpha loading factor in Xenopus. Arabidopsis cdc45 is up-regulated at the G1/S transition and in young meiotic flower buds. Arabidopsis cdc45 RNAi lines are partially to completely sterile. The severity of the phenotype is correlated to levels of the cdc45 transcript and small RNA fragments. Severe chromosome fragmentation arising during meiosis leads to abnormal chromosome segregation and unequal distribution of meiotic products resulting in defective pollen and ovule development. Microarray analysis as been done with Meyer's group on 35S::RNAiCDC45 plants. Meyer's group reported an increased expression of HAP5A, 2A, 4A and CDC2a and b as well as down regulation of cdc6, and E2F1 (E2Fa).

The commitment to DNA replication is a key step in cell division control. Consistent with its role at the G1/S transition the AtMCM3 gene is transcriptionally regulated at S phase. The 5' region of this gene contains several E2F consensus-binding sites, two of which match the human consensus closely.

Furthermore the promoter is activated by AtE2F-a and AtDP-a factors in transient expression studies while mutating either E2F binding site decreases the activity of the promoter. One of the E2F binding sites is shown to be responsible for the G2-specific repression of the promoter in synchronised cell suspension cultures. The second E2F binding site has a role in meristem-specific expression in planta as deletion of this site eliminates the expression of a reporter gene in root and apical meristems. Thus two highly similar E2F binding sites in the promoter of the MCM3 gene are responsible for different cell cycle regulation or developmental expression patterns depending on the cellular environment (Stevens et al., 2002).

Bergounioux's group showed that MCM2 or MCM3 anti-sense constructs did not give any phenotypes. CropDesign showed that plant over-pressing At MCM2 or 3 had reduced growth.

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CNRS Institut de Biologie des Plantes
91405 Orsay
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