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A protein factor, identified in nuclear extracts obtained from tomato (Lycopersicon esculentum, Solanaceae) and Arabidopsis thaliana (Brassicaceae) seedlings, specifically binds upstream sequences from the plant light-regulated gene family encoding the small subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase (RBCS). RBCS upstream sequences from tomato, pea (Pisum sativum, Leguminosae), and Arabidopsis are recognized by the factor. The factor recognition occurs via a short conserved sequence (G box) whose consensus sequence is 5'-TCTTACACGTGGCAYY-3' (where Y is pyrimidine). This sequence is distinct from the GT motif described previously in RBCS promoters. Two other conserved sequences, showing a lesser degree of evolutionary conservation, are found upstream of the G box but do not bind to the G box binding factor (GBF). Twelve nucleotides within the G box are sufficient for …