The Myb domain of LUX ARRHYTHMO in complex with DNA: expression, purification and crystallization

Abundant clock proteins point to missing molecular regulation in the plant circadian clock

Understanding the biochemistry behind whole-organism traits such as flowering time is a longstanding challenge, where mathematical models are critical. Very few models of plant gene circuits use the absolute units required for comparison to biochemical data. We refactor two detailed models of the plant circadian clock from relative to absolute units. Using absolute RNA quantification, a simple model predicted abundant clock protein levels in Arabidopsis thaliana, up to 100,000 proteins per cell. NanoLUC reporter protein fusions validated the predicted levels of clock proteins in vivo. Recalibrating the detailed models to these protein levels estimated their DNA-binding dissociation constants (Kd). We estimate the same Kd from multiple results in vitro, extending the method to any promoter sequence. The detailed models simulated the Kd range estimated from LUX DNA-binding in vitro but departed from the data for CCA1 binding, pointing to further circadian mechanisms. Our analytical and experimental methods should transfer to understand other plant gene regulatory networks, potentially including the natural sequence variation that contributes to evolutionary adaptation.

The LUX-SWI3C module regulates photoperiod sensitivity in Arabidopsis thaliana

In plants, the photoperiod sensitivity directly influences flowering time, which in turn affects latitudinal adaptation and yield. However, research into the mechanisms underlying photoperiod sensitivity, particularly those mediated by epigenetic regulation, is still in its nascent stages. In this study, we analyzed the regulation of photoperiod sensitivity in Arabidopsis thaliana. We demonstrate that the evening complex LUX ARRYTHMO (LUX) and the chromatin remodeling factor SWITCH/SUCROSE NONFERMENTING 3C (SWI3C) regulate GI locus chromatin compaction and H3K4me3 modification levels at the GIGANTEA locus under different photoperiod conditions. This mechanism is one of the key factors that allow plants to distinguish between long-day and short-day photoperiods. Our study provides insight into how the LUX-SWI3C module regulates photoperiod sensitivity at the epigenetic level.

Plant-TFClass: a structural classification for plant transcription factors

Transcription factors (TFs) bind DNA at specific sequences to regulate gene expression. This universal process is achieved via their DNA-binding domain (DBD). In mammals, the vast diversity of DBD structural conformations and the way in which they contact DNA has been used to organize TFs in the TFClass hierarchical classification. However, the numerous DBD types present in plants but absent from mammalian genomes were missing from this classification. We reviewed DBD 3D structures and models available for plant TFs to classify most of the 56 recognized plant TF types within the TFClass framework. This extended classification adds eight new classes and 37 new families corresponding to DBD structures absent in mammals. Plant-TFClass provides a unique resource for TF comparison across families and organisms.

Molecular mechanisms of Evening Complex activity in Arabidopsis

Circadian gene expression oscillates over a 24-h period and regulates many genes critical for growth and development in plants. A key component of the circadian clock is the Evening Complex (EC), a transcriptional repressor complex that contains the proteins LUX ARRHYTHMO, EARLY FLOWERING 3, and EARLY FLOWERING 4 (ELF4). By repressing the expression of genes such as PHYTOCHROME INTERACTING FACTOR4 (PIF4), the EC reduces elongation growth. At warmer temperatures, EC activity is lost, promoting thermomorphogenesis via PIF4 expression. The molecular mechanisms underlying EC activity are not well understood. Here, we combined structural studies with extensive in vitro assays to determine the molecular mechanisms of the temperature-dependent EC binding to DNA and demonstrate the critical role of ELF4 in this activity.

The Evening Complex (EC), composed of the DNA binding protein LUX ARRHYTHMO (LUX) and two additional proteins EARLY FLOWERING 3 (ELF3) and ELF4, is a transcriptional repressor complex and a core component of the plant circadian clock. In addition to maintaining oscillations in clock gene expression, the EC also participates in temperature and light entrainment, acting as an important environmental sensor and conveying this information to growth and developmental pathways. However, the molecular basis for EC DNA binding specificity and temperature-dependent activity were not known. Here, we solved the structure of the DNA binding domain of LUX in complex with DNA. Residues critical for high-affinity binding and direct base readout were determined and tested via site-directed mutagenesis in vitro and in vivo. Using extensive in vitro DNA binding assays of LUX alone and in complex with ELF3 and ELF4, we demonstrate that, while LUX alone binds DNA with high affinity, the LUX–ELF3 complex is a relatively poor binder of DNA. ELF4 restores binding to the complex. In vitro, the full EC is able to act as a direct thermosensor, with stronger DNA binding at 4 °C and weaker binding at 27 °C. In addition, an excess of ELF4 is able to restore EC binding even at 27 °C. Taken together, these data suggest that ELF4 is a key modulator of thermosensitive EC activity.

Structural basis for the Target DNA recognition and binding by the MYB domain of phosphate starvation response 1

The phosphate starvation response 1 (PHR1) protein has a central role in mediating the response to phosphate starvation in plants. PHR1 is composed of a number of domains including a MYB domain involved with DNA binding and a coiled-coil domain proposed to be involved with dimer formation. PHR1 binds to the promoter of phosphate starvation-induced genes to control the levels of phosphate required for nutrition. Previous studies have shown that both the MYB domain and the coiled-coil domain of PHR1 are required for binding the target DNA. Here, we describe the crystal structure of the PHR1 MYB domain and two structures of its complex with the PHR1-binding DNA sequence (P1BS). Structural and isothermal titration calorimetry has been carried out showing that the MYB domain of PHR1 alone is sufficient for target DNA recognition and binding. Two copies of the PHR1 MYB domain bind to the same major groove of the P1BS DNA with few direct interactions between the individual MYB domains. In addition, the PHR1 MYB-P1BS DNA complex structures reveal amino acid residues involved in DNA recognition and binding. Mutagenesis of these residues results in lost or impaired ability of PHR1 MYB to bind to its target DNA. The results presented reveal the structural basis for DNA recognition by the PHR1 MYB domain and demonstrate that two PHR1 MYB domains attach to their P1BS DNA targeting sequence. DATABASE: Coordinates and structure factors have been deposited in the Protein Data Bank under accession codes 6J4K (PHR1 MYB), 6J4R (PHR1 MYB-R-P1BS), 6J5B (MYB-CC-R2-P1BS).

Merging of synchrotron serial crystallographic data by a genetic algorithm

Recent advances in macromolecular crystallography have made it practical to rapidly collect hundreds of sub-data sets consisting of small oscillations of incomplete data. This approach, generally referred to as serial crystallography, has many uses, including an increased effective dose per data set, the collection of data from crystals without harvesting (in situ data collection) and studies of dynamic events such as catalytic reactions. However, selecting which data sets from this type of experiment should be merged can be challenging and new methods are required. Here, it is shown that a genetic algorithm can be used for this purpose, and five case studies are presented in which the merging statistics are significantly improved compared with conventional merging of all data.