LNK1 and LNK2 Are Transcriptional Coactivators in the Arabidopsis Circadian Oscillator

Abstract

Transcriptional feedback loops are central to the architecture of eukaryotic circadian clocks. Models of the Arabidopsis thaliana circadian clock have emphasized transcriptional repressors, but recently, Myb-like REVEILLE (RVE) transcription factors have been established as transcriptional activators of central clock components, including PSEUDO-RESPONSE REGULATOR5 (PRR5) and TIMING OF CAB EXPRESSION1 (TOC1). We show here that NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED1 (LNK1) and LNK2, members of a small family of four LNK proteins, dynamically interact with morningexpressed oscillator components, including RVE4 and RVE8. Mutational disruption of LNK1 and LNK2 function prevents transcriptional activation of PRR5 by RVE8. The LNKs lack known DNA binding domains, yet LNK1 acts as a transcriptional activator in yeast and in planta. Chromatin immunoprecipitation shows that LNK1 is recruited to the PRR5 and TOC1 promoters in planta. We conclude that LNK1 is a transcriptional coactivator necessary for expression of the clock genes PRR5 and TOC1 through recruitment to their promoters via interaction with bona fide DNA binding proteins such as RVE4 and RVE8.

Time-dependent sequestration of RVE8 by LNK proteins shapes the diurnal oscillation of anthocyanin biosynthesis

“…S5). The yeast two-hybrid screening is thus consistent with a previous report showing the rhythmic interaction of LNK1 and LNK2 with RVE8 and with RVE4 (21).…”

“…The fact that under LD cycles, the anthocyanin biosynthetic genes were up-regulated in both RVE8-ox and dm plants also suggests that RVE8 might act as an activator, whereas LNK1 and LNK2 might be repressors of the anthocyanin biosynthetic pathway. A previous report has demonstrated that LNK1 and LNK2 together with RVE4 and RVE8 act as transcriptional coactivators in the regulation of circadian gene expression (21). Our results open the interesting possibility that under LD cycles, the role of LNK-RVE8 interaction in the control of anthocyanin regulation might be opposed to that exerted on circadian core gene expression.…”

“…S4). Similar to the RVE8 oscillation, the expression of LNKs rhythmically oscillate under LD cycles and under LL conditions with a peak close to dawn (20)(21)(22). Analysis of the RVE8 coexpressed gene network uncovered the members of the LNK family as highly significant genes coexpressed with RVE8 (SI Appendix, Fig.…”

“…RVE8 is able to directly bind to the promoters of its target circadian genes (15,17,21). Therefore, we next investigated whether RVE8 binds in vivo to the promoters of the anthocyanin biosynthetic genes.…”

“…Recent studies have identified the NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED genes (LNK) as key circadian components with an important role in seasonal adjustment (20). The LNKs integrate environmental signals to control the expression of afternoon genes, allowing plants to perceive and respond to seasonal changes in day length and temperature (20)(21)(22). Two members of the LNK family (LNK1 and LNK2) interact with RVE8 and RVE4 and form a protein complex that is important for the transcriptional activation of the evening-phased clock genes, PRR5 and TOC1 (21).…”