The role of the F-box protein UFO in regulation of the floral homeotic APETALA3 gene

UNUSUAL FLORAL ORGANS (UFO) is an upstream regulator of the floral homeotic APETALA3 (AP3) gene that specifies petal and stamen organ identities. Here, I present data that elucidates the molecular mechanisms by which UFO functions to activate AP3 expression in a spatio-temporal specific manner. UFO has been proposed to function as a cofactor of the transcription factor LFY for activating AP3 expression (Lee et al., 1997; Parcy et al., 1998). In this study, I demonstrated that UFO directly binds to LFY and associates with the AP3 promoter through the interaction with LFY, although UFO encodes an F-box box protein without DNA-binding capability. I have also shown that a repressor domain fusion to UFO can reduce UFO activity with regard to activating AP3 expression. These results indicate that the F-box protein UFO functions as a transcriptional cofactor of LFY for AP3 activation. In addition, using inducible form of LFY, I have shown that proteasome activity is required for LFY and UFO to induce AP3 expression, indicating that SCFUFO mediated proteasomal degradation of a factor at the AP3 promoter may indeed promote AP3 expression. Based on these observations I present two working hypotheses of UFO action as an example of how ubiqutin E3 ligases can activate transcription.; In addition to a LFY-dependent mode of action, I have uncovered another functional aspect of UFO in ubiquitin signaling by identifying new UFO interacting factors. These include ubiquitin conjugating enzymes and a 14-3-3 protein, which may function to regulate the activity of SCFUFO. In addition, I isolated a LIM protein that interacts with UFO and can also bind to ubiquitin through ubiquitin interacting motifs (UIMs) at its N-terminal end, suggesting that LIM may function as an ubiquitin receptor. Furthermore, my genetic analyses combined with interaction assays and repressor assays suggests that LIM functions as a putative transcriptional corepressor of AP3 expression. These results suggest that SCFUFO mediated ubiquitination of the LIM protein may modulate its corepressor activity to give rise to derepression of the AP3 promoter. Together, these data shed light on new modes of action for the F-box protein UFO in regulating floral development.