The water- and salt-stress-regulated Asr1 (abscisic acid stress ripening) gene encodes a zinc-dependent DNA-binding protein

Tomato (Lycopersicon esculantum) ASRI (abscisic acid stress ripening protein), a small plant-specific protein whose cellular mode of action defies deduction based on its sequence or homology analyses, is one of numerous plant gene products with unknown biological roles that become over-expressed under water- and salt-stress conditions. Steady-state cellular levels of tomato ASRI mRNA and protein are transiently increased following exposure of plants to poly(ethylene glycol), NaCl or abscisic acid. Western blot and indirect immunofluorescence analysis with antiASRI antibodies demonstrated that ASRI is present both in the cytoplasmic and nuclear subcellular compartments; approx. one-third of the total ASRI protein could be detected in the nucleus. Nuclear ASRI is a chromatin-bound protein, and can be extracted with I M NaCl, but not with 0.5% Triton X-100. ASRI, over-expressed in Escherichia coli and purified to homogeneity, possesses zinc-dependent DNA-binding activity. Competitive-binding experiments and SELEX (systematic evolution of ligands by exponential enrichment) analysis suggest that ASRI binds at a preferred DNA sequence.