Maize E2F transcription factors. Expression, association to promoters of S-phase genes and interaction with the RBR1 protein in chromatin during seed germination.

For seed germination, it is necessary to restart the cell cycle, a process regulated at multiple levels including transcriptional control, that is executed by the E2F family of transcription factors. We identified 12 genes of the E2F family in maize that are expressed differentially during the first 28 h post imbibition (HAI). E2Fa/b1;1 and E2Fc proteins were characterized as an activator and a putative repressor respectively, both forming heterodimers with DPb2 that bind differentially to consensus E2F response elements in promoters of E2F target genes. Transcripts of target genes for these transcription factors accumulate during germination; in dry seeds E2Fc protein is enriched in the target promoters and is replaced by E2Fa/b1;1 as germination advances. RBR1 is found in the same promoters in non-imbibed and 28 HAI seeds, when DNA replication has concluded, and transcription of the E2F targets should stop. During germination promoters of these target genes seem to be decorated with histone marks related to relaxed chromatin structure. Therefore, E2Fs appear to occupy their target genes in a context of open chromatin, with RBR1 fine tuning the progression between the phases.