Deciphering the molecular mechanisms underpinning the transcriptional control of gene expression by L-AFL proteins in Arabidopsis seed.
Résumé
In Arabidopsis (Arabidopsis thaliana), transcriptional control of seed maturation involves three related regulators with a B3
domain, namely LEAFY COTYLEDON2 (LEC2), ABSCISIC ACID INSENSITIVE3 (ABI3), and FUSCA3 (ABI3/FUS3/LEC2
[AFLs]). Although genetic analyses have demonstrated partially overlapping functions of these regulators, the underlying
molecular mechanisms remained elusive. The results presented here confirmed that the three proteins bind RY DNA elements
(with a 59-CATG-39 core sequence) but with different specificities for flanking nucleotides. In planta as in the moss Physcomitrella
patens protoplasts, the presence of RY-like (RYL) elements is necessary but not sufficient for the regulation of the OLEOSIN1 (OLE1)
promoter by the B3AFLs. G box-like domains, located in the vicinity of the RYL elements, also are required for proper activation of
the promoter, suggesting that several proteins are involved. Consistentwith this idea, LEC2 andABI3 showed synergistic effects on
the activation of the OLE1 promoter. What is more, LEC1 (a homolog of the NF-YB subunit of the CCAAT-binding complex)
further enhanced the activation of this target promoter in the presence of LEC2 and ABI3. Finally, recombinant LEC1 and LEC2
proteins produced in Arabidopsis protoplasts could form a ternary complex with NF-YC2 in vitro, providing a molecular explanation
for their functional interactions. Taken together, these results allow us to propose a molecularmodel for the transcriptional regulation
of seed genes by the L-AFL proteins, based on the formation of regulatory multiprotein complexes between NF-YBs, which carry a
specific aspartate-55 residue, and B3 transcription factors.