Rubisco (EC 1.1.1.39) is a key enzyme in photosynthetic carbon assimilation and catalyzes the first committed step of CO2 fixation in the Calvin cycle. It has been estimated that nearly 70% of soluble leaf protein is in the form of Rubisco (4). Higher plant Rubisco is an oligomer composed of eight large subunits (Mr 52,000-55,000) and eight small subunits (Mr 12,000-15,000). The large subunit contains the active site for carboxylation and oxygenation reactions, and the small subunit protein is hypothesized to maintain the apoenzyme in a stable conformation capable of activation to a catalytically active holoenzyme (5). The small subunit protein of Rubisco is nuclear encoded by the rbcS gene which exists as a multigene family (2). In contrast, the Rubisco large subunit protein is encoded by the rbcL gene situated on the chloroplastic DNA (5). The rbcL gene exists as a single copy and contains no introns. Currently, no information exists concerning the rbcL nucleotide sequence of potato (Solanum tuberosum). The objective of this study was to investigate rbcL to identify differences in the nucleotide sequence for Rubisco from cultivars of potato previously determined to be sensitive (cv Cherokee) and tolerant (cv Superior) to ozone (1, 6). The mechanism for differential genotypic responses to ozone is unknown. One explanation may be the vulnerability of SH side groups of Rubisco to oxidation (9). The oxidation of SH groups is known to render Rubisco catalytically inactive and more susceptible to proteolysis (7). Chloroplast DNA was isolated from both potato cultivars and screened with the rbcL gene from maize (Table I). The nucleotide and predicted amino acid sequences for rbcL from potato cultivars Cherokee and Superior were found to be identical (Fig. 1). Consequently, the differential sensitivity to ozone between cultivars could not be explained by the absolute number of Cys residues because each nucleotide sequence predicts the occurrence of nine SH groups per large subunit protein (Fig. 1). For comparison purposes, the deduced amino acid sequence of tobacco (8) and spinach (10) rbcL genes are shown (Fig. 1). It is still possible that the difference in the Rubisco response can be explained by characteristics of the rbcS gene(s) and/or expression of these genes. Table I. Characteristics of Gene rbcL from Solanum tuberosum Chloroplast