Proteasome-Mediated Turnover of Arabidopsis MED25 Is Coupled to the Activation of FLOWERING LOCUS T Transcription  

Abstract

The Mediator complex is a greater than 1-megadalton complex, composed of about 30 subunits and found in most eukaryotes, whose main role is to transmit signals from DNA-bound transcription factors to RNA Polymerase II. The proteasome is emerging as an important regulator of transcription during both initiation and elongation. It is increasing the number of cases where the proteolysis of transcriptional activators by the proteasome activates their function. This counterintuitive phenomenon was called “activation by destruction.” Here, we show that, in Arabidopsis (Arabidopsis thaliana), PHYTOCHROME AND FLOWERING TIME1 (PFT1), the MEDIATOR25 (MED25) subunit of the plant Mediator complex, is degraded by the proteasome and that proteasome-mediated PFT1 turnover is coupled to its role in stimulating the transcription of FLOWERING LOCUS T, the plant florigen, which is involved in the process of flowering induction. We further identify two novel RING-H2 proteins that target PFT1 for degradation. We show that MED25-BINDING RING-H2 PROTEIN1 (MBR1) and MBR2 bind to PFT1 in yeast (Saccharomyces cerevisiae) and in vitro, and they promote PFT1 degradation in vivo, in a RING-H2-dependent way, typical of E3 ubiquitin ligases. We further show that both MBR1 and MBR2 also promote flowering by PFT1-dependent and -independent mechanisms. Our findings extend the phenomenon of activation by destruction to a Mediator subunit, adding a new mechanism by which Mediator subunits may regulate downstream genes in specific pathways. Furthermore, we show that two novel RING-H2 proteins are involved in the destruction of PFT1, adding new players to this process in plants.

HEMERA Couples the Proteolysis and Transcriptional Activity of PHYTOCHROME INTERACTING FACTORs in Arabidopsis Photomorphogenesis

“…MED25 interacts directly with the transcriptional activation domain of VP16 (Vojnic et al, 2011). The ortholog of MED25 in Arabidopsis, known as PFT1 (Phytochrome and Flowering Time1) (Cerdán and Chory, 2003;Bäckström et al, 2007), was identified as a factor involved in phy signaling and flowering time (Cerdán and Chory, 2003;Iñigo et al, 2012aIñigo et al, , 2012bKlose et al, 2012). Our future investigation will test the hypothesis that HMR's 9aaTAD interacts directly with PFT1 to promote gene activation.…”

PHYTOCHROME-DEPENDENT LATE-FLOWERING accelerates flowering through physical interactions with phytochrome B and CONSTANS

“…Red light and blue light play antagonistic roles in the regulation of CO mRNA transcription. In response to red light, phyB down-regulates CO mRNA transcription through factors such as PHYTOCHROME AND FLOWERING TIME 1 (8)(9)(10). In contrast, cry2 and phyA up-regulate CO mRNA transcription (7,11).…”

Photoperiodic Regulation of Florigen Function inArabidopsis thaliana

“…Genetic evidence suggests that PFT1 serves an activating role both in CO transcription as well as FT transcription, through independent mechanisms (Inigo et al, 2012b). At least at FT, PFT1 is degraded by the proteasome through the action of two interacting proteins MED25-BINDING RING-H2 PROTEIN1 (MBR1: At2g15530) and MRB2 (At4g34040) (Inigo et al, 2012a). Degradation of PFT1 is important for activation of FT, as inhibition of PFT1 degradation prevents FT activation.…”