S Phase Progression Is Required for Transcriptional Activation of the -Phaseolin Promoter*□S

Elucidating the mechanisms by which the transcription machinery accesses promoters in their chromatin environment is a fundamental aspect of understanding gene regulation. The phas promoter is normally constrained by a rotationally and translationally positioned nucleosome over its TATA region except during embryogenesis when it is potentiated by the presence of Phaseolus vulgaris ABI3-like factor (PvALF), a plantspecific transcription factor, and activated by an abscisic acid (ABA)-induced signal transduction cascade. Ectopic expression of PvALF and the supply of ABA in transgenic tobacco or Arabidopsis leaves can activate expression from phas. We confirmed by [H]thymidine incorporation that active DNA replication occurred concomitant with the presence of PvALF and ABA. Arrest of DNA synthesis or S phase progression by infiltration of the leaves with replication inhibitors (hydroxyurea, roscovitine, mimosine) strongly inhibited transcriptional activation, especially the ABA-mediated activation step. Similarly, activation of endogenous Arabidopsis MAT and LEA genes in leaf tissue by the presence of ABA and ectopically expressed PvALF was inhibited by DNA replication arrest. No change in transcript levels on the arrest of replication was detected for abi1, abi2, and era1, negative regulators of the ABA signal transduction cascade or for cell cycle components ick1 and aip3. However, a reduction in transcript accumulation for the crucial ABA signaling effector, abi5, occurred upon DNA replication arrest (probably reflected in the decrease in MAT and LEA gene expression). Contrary to the conventional view that ABA inhibits DNA replication, our findings show that ABA acts in concert with S phase progression to activate gene expression.