Guo, Mengjuan , Julie, Thomas , Galen, Collins , Marja, Timmermans .
Molecular Mechanisms for KNOX Gene Silencing by the Arabidopsis ASYMMETRIC LEAVES1 Complex.
Plant shoots maintain indeterminate growth resulting from the action of a population of stem cells in the shoot apical meristem (SAM). Class I knox homeobox genes are among the key factors that specify indeterminacy in the SAM. Down-regulation of the knox genes is required for leaf initiation and for the establishment of determinant organs. In Arabidopsis thaliana, the latter process is mediated by the Myb domain protein ASYMMETRIC LEAVES1 (AS1).
We found that a fusion of the AS1 non-Myb domain to the DNA binding domain of LEAFY, a transcriptional factor required for floral induction, blocked the activation of LEAFY targets, suggesting AS1 is a transcriptional co-repressor that acts directly at the knox targets. Consistent with this hypothesis, we recently showed that AS1 is part of a cellular memory system that also includes the DNA binding protein ASYMMETRIC LEAVES2 (AS2), a predicted RNA binding protein RIK, and a homologue of the chromatin-remodeling factor HIRA.
Using biochemical and genetic approaches we have defined the cis- and trans-acting factors that mediate the recruitment of the AS1 complex to the knox targets. Using chromatin immunoprecipitation (ChIP), we have detected three sites in the knox gene BREVIPEDICELLUS (BP) that are directly bound to AS1 protein complex. Two sites are in the promoter region. Our BP promoter deletion analysis verified the significance of the two ChIP identified sites for AS1-mediated BP silencing. Data from eletrophoretic mobility shift assay further defined the cis-regulatory sequences recruiting AS1 complex, also indicated that the interaction between AS1 and AS2 is required for their binding to the BP promoter. Furthermore, we identified an antisense RNA from the third ChIP binding site, and characterized its functions for the BP silencing and leaf development in artificial miRNA knock out lines.
Together, our data suggests a molecular framework for maintaining determinacy during organogenesis. Interaction between AS1 and AS2 facilitates their binding to the knox targets and the recruitment of the chromatin-remodeling protein HIRA, which maintains determinacy perhaps by forming a stable repressive chromatin state at the loci.
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1 - Cold Spring Harbor Laboratory, 1 Bungtown Rd., Cold Spring Harbor, NY, 11743, USA
2 - Cold Spring Harbor Laboratory
3 - Watson School of Biological Sciences
Presentation Type: ASPB Minisymposium
Location: Continental B/Hilton
Date: Sunday, July 8th, 2007
Time: 2:50 PM