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SYSTEMS BIOLOGY, MOLECULAR BIOLOGY, AND GENE REGULATION

FRIGIDA Delays Flowering in Arabidopsis via a Cotranscriptional Mechanism Involving Direct Interaction with the Nuclear Cap-Binding Complex^1,[W]

Department of Cell and Developmental Biology, John Innes Centre, Norwich NR4 7UH, United Kingdom (N.G., I.B., X.H., C.D.); and Cryobiosystem Research Center, Iwate University, Morioka, Iwate 020–8550, Japan (S.-i.K.)

A major determinant of flowering time in natural Arabidopsis (Arabidopsis thaliana) variants is FRIGIDA (FRI). FRI up-regulates expression of the floral repressor FLOWERING LOCUS C (FLC), thereby conferring a vernalization requirement and a winter annual habit. FRI encodes a novel nuclear protein with no conserved domains except for two coiled-coil regions. A mutation in the large subunit of the nuclear cap-binding complex (CBC) suppresses FRI activity, so we have explored the connection between FRI and the nuclear CBC in order to gain further insight into FRI biochemical activity. Mutations in the small subunit of the CBC (CBP20) also suppress FRI up-regulation of FLC. CBP20 interacted directly with FRI in yeast and in planta, and this association of FRI with the 5' cap was reinforced by an RNA ligase-mediated rapid amplification of cDNA ends assay that showed FRI decreased the proportion of FLC transcripts lacking a 5' cap. Loss of CBP20 resulted in very low FLC mRNA levels and an increased proportion of unspliced FLC transcripts. FRI compensated for CBP20 loss, partially restoring FLC levels and normalizing the unspliced-spliced transcript ratio. Our data suggest that FRI up-regulates FLC expression through a cotranscriptional mechanism involving direct physical interaction with the nuclear CBC with concomitant effects on FLC transcription and splicing.

² Present address: Instituto de Tecnologia Química e Biológica, Avenida da República, Estação Agronómica Nacional, 2780–157 Oeiras, Portugal.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Caroline Dean (caroline.dean{at}bbsrc.ac.uk).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.137448

^* Corresponding author; e-mail caroline.dean{at}bbsrc.ac.uk.

Received February 21, 2009; accepted May 3, 2009; published May 8, 2009.

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