TRANSCRIPTIONAL REGULATION OF ENVIRONMENTALLY INDUCIBLE GENES IN PLANTS BY AN EVOLUTIONARY CONSERVED FAMILY OF G-BOX BINDING-FACTORS

1. In reviewing a number of the most intensely studied environmentally inducible promoters it becomes clear that the presence of two cis-acting elements are critical for promoter activity, one of which is the G-box (CCACGTGG). A mutation in one of the two elements abolishes or severely reduces the ability of the promoter to respond to environmental changes. The sequence of the second cis-acting element, positioned nearby the G-box, is not conserved among the different inducible promoters, but may be similar among promoters induced by the same signal. The spacing between the G-box and the second cis-acting element appears to be critical, suggesting a direct interaction between the respective binding factors. We speculate on a potential role of the G-box promoter element in the signal induction of promoter activity. 2. From a number of plant species nuclear proteins interacting with the G-box have been identified. Recently, G-box Binding Factors (GBF) have been isolated by screening cDNA expression libraries with a characterized G-box cis-acting element as DNA probe. The deduced amino acid sequence of the GBF clones revealed that they possess the features of the basic leucine zipper class of trans-acting factors. By amino acid sequence comparison and limited mutational analysis, we define amino acids critical for G-box binding specificity. All GBFs isolated to date have a conserved proline-rich domain involved in transcriptional activation. A number of GBFs are inducible by a particular environmental signal. 3. Recently, a protein designated GF14 has been isolated that is associated with the GBF protein complex. The protein has homology to mammalian brain specific proteins, which seem to function as regulators of phosphorylation events. GBF activity is regulated by phosphorylation. The GF14 proteins may therefore impose an additional control on gene expression.