CONSTITUTIVE, LIGHT-RESPONSIVE AND CIRCADIAN CLOCK-RESPONSIVE FACTORSCOMPETE FOR THE DIFFERENT L-BOX ELEMENTS IN PLANT LIGHT-REGULATED PROMOTERS

The I box is a conserved regulatory motif which is found upstream of p lant genes (rbcS, cab and nia) whose transcription is regulated by lig ht and the circadian clock. Gel retardation and UV cross-linking assay s 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 I box of the light-responsive rbcS promoter; on e factor within this group, IBF-1a, also recognizes the adjacent G box , which has been shown previously to bind a different class of plant t ranscription factors, the G box binding factors (GBFs). To the limit o f experimental resolution, IBF-1a and GBF compete for the same nucleot ides on the G box. Nevertheless, these two activities are biochemicall y 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 I box of the nia promoter, which is regulated bo th by light and the circadian clock; one factor within this group, IBF -2a, also binds the I box of a second promoter showing similar regulat ion, the cab promoter. The IBF-2a binding sites on the cab and nia pro moters show extensive homology to a circadian clock-responsive promote r element from wheat. The abundance of IBF-2a is diurnally regulated a nd shows a dramatic induction around the onset of the light period. Tr ansfer of the plants in continuous darkness demonstrates that this ind uction is under the control of a circadian clock. These data suggest t hat I box binding factors may be involved in regulation of transcripti on by light and the circadian clock.