title: Constitutive, light-responsive and circadian clock-responsive factors compete for the different l box elements in plant light-regulated promoters.
abstract: The l box is a conserved regulatory motif which is found upstream of plant genes (rbcS, cab and nia) whose transcription is regulated by light and the circadian clock. Gel retardation and UV cross-linking assays were used to resolve two different groups of I box binding factors (IBFs) in tomato nuclear extracts. Active components of the first group (IBF-1) recognize the l box of the light-responsive rbcS promoter; one factor within this group, IBF-1a, also recognizes the adjacent G box, which has been shown previously to bind a different class of plant transcription factors, the G box binding factors (GBFs). To the limit of experimental resolution, IBF-1a and GBF compete for the same nucleotides on the G box. Nevertheless, these two activities are biochemically and immunologically distinct. The relative abundance of IBF-1a shows a vast decrease in dark-adapted plants. Factors in the second group (IBF-2), recognize the l box of the nia promoter, which is regulated both by light and the circadian clock; one factor within this group, IBF-2a, also binds the l box of a second promoter showing similar regulation, the cab promoter. The IBF-2a binding sites on the cab and nia promoters show extensive homology to a circadian clock-responsive promoter element from wheat. The abundance of IBF-2a is diurnally regulated and shows a dramatic induction around the onset of the light period. Transfer of the plants in continuous darkness demonstrates that this induction is under the control of a circadian clock.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: ENEA, CRE Casaccia, Roma, Italy.
authors.forename: U
authors.initials: U
authors.lastname: Borello
authors.forename: E
authors.initials: E
authors.lastname: Ceccarelli
authors.forename: G
authors.initials: G
authors.lastname: Giuliano
mesh_terms: D001483:Base Sequence; D001667:Binding, Competitive; D002940:Circadian Rhythm; D004247:DNA; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation; D008027:Light; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000528:radiation effects; D012045:Regulatory Sequences, Nucleic Acid; D014675:Vegetables / Q000235:genetics*
chemical_list: D004268:DNA-Binding Proteins; D010940:Plant Proteins; D004247:DNA