title: Structural and biochemical characterization of a cyanobacterium circadian clock-modifier protein.
abstract: Circadian clocks are self-sustained biochemical oscillators. The oscillator of cyanobacteria comprises the products of three kai genes (kaiA, kaiB, and kaiC). The autophosphorylation cycle of KaiC oscillates robustly in the cell with a 24-h period and is essential for the basic timing of the cyanobacterial circadian clock. Recently, period extender (pex), mutants of which show a short period phenotype, was classified as a resetting-related gene. In fact, pex mRNA and the pex protein (Pex) increase during the dark period, and a pex mutant subjected to diurnal light-dark cycles shows a 3-h advance in rhythm phase. Here, we report the x-ray crystallographic analysis and biochemical characterization of Pex from cyanobacterium Synechococcus elongatus PCC 7942. The molecule has an (alpha+beta) structure with a winged-helix motif and is indicated to function as a dimer. The subunit arrangement in the dimer is unique and has not been seen in other winged-helix proteins. Electrophoresis mobility shift assay using a 25-base pair complementary oligonucleotide incorporating the kaiA upstream sequence demonstrates that Pex has an affinity for the double-stranded DNA. Furthermore, mutation analysis shows that Pex uses the wing region to recognize the DNA. The in vivo rhythm assay of Pex shows that the constitutive expression of the pex gene harboring the mutation that fails to bind to DNA lacks the period-prolongation activity in the pex-deficient Synechococcus, suggesting that Pex is a DNA-binding transcription factor.
authors.affiliation: International Graduate School of Arts and Sciences, Yokohama City University, 1-7-29 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan.
authors.forename: Kyouhei
authors.initials: K
authors.lastname: Arita
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Hashimoto
authors.forename: Kumiko
authors.initials: K
authors.lastname: Igari
authors.forename: Mayuko
authors.initials: M
authors.lastname: Akaboshi
authors.forename: Shinsuke
authors.initials: S
authors.lastname: Kutsuna
authors.forename: Mamoru
authors.initials: M
authors.lastname: Sato
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Shimizu
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics*; D002940:Circadian Rhythm / Q000235:genetics*; D017124:Conserved Sequence; D003461:Crystallography; D004247:DNA / Q000378:metabolism; D004269:DNA, Bacterial / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics; D019281:Dimerization; D024202:Electrophoretic Mobility Shift Assay; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D046940:Synechococcus / Q000235:genetics*; D051540:Winged-Helix Transcription Factors / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D004268:DNA-Binding Proteins; C111941:Pex protein, Synechococcus; D051540:Winged-Helix Transcription Factors; D004247:DNA