The assembly of the CAAT-box binding complex at a photosynthesis gene promoter is regulated by light, cytokinin, and the stage of the plastids.
A functionally important region in the promoter of the spinach photosynthesis gene AtpC, which encodes the subunit gamma of the chloroplast ATP synthase, is located immediately upstream of the CAAT-box. A single nucleotide exchange in this region (AAAATTCAAT --> AAGATCAAT) uncouples the expression of an AtpC promoter::uidA gene fusion from the regulation by light, cytokinin, and functional plastids and results in a high constitutive expression of the reporter gene. By screening an Arabidopsis thaliana expression library with a double-stranded wild-type oligonucleotide from this promoter region, we have isolated cDNAs from Arabidopsis libraries that code for plant homologs of the CAAT-box binding factor (CBF)-C. Binding occurs only in the presence of nuclear extracts, consistent with reports from metazoa CBFs that the subunits A and B in addition to C are required for the formation of the CBF-DNA complex. At least eight genes with homologies to CBF-C are present in the Arabidopsis genome; one of them exhibits striking similarities to the gene for the human global transcriptional repressor Drap1. In gel mobility shift assays, low binding activity of CBF to the wild-type AtpC promoter sequence was observed with nuclear extracts from tissue with low AtpC expression levels, i.e. extracts from etiolated and photobleached seedlings, whereas high binding activity was detectable with extracts from tissues with high AtpC expression levels, i.e. extracts from light-grown seedlings and etiolated seedlings treated with cytokinin. Binding to the mutant sequence, which directs constitutive high level uidA expression in vivo, is significantly stronger than to the wild-type sequence. The data are consistent with the idea that the assembly of CBF at the AtpC promoter is regulated in response to light and cytokinin and that the low level of expression in etiolated and photobleached material is caused by an inhibitory effect. The structure/function relationships of the Arabidopsis CBFs are discussed in relation to their regulatory function in AtpC gene expression.