APx (EC 1.11.1.11) is a heme-containing, nonglycosylated enzyme that destroys harmful hydrogen peroxide via the ascorbate-glutathione pathway in plants. This pathway provides protection against oxidative stress by a series of coupled redox reactions, particularly in photosynthetic tissues (Foyer and Halliwell, 1976). Three types of APx are known in plants: a cytosolic APx, a chloroplastic stromal APx, and a thylakoid membrane-bound APx. They differ in mo1 wt, substrate specificity, pH optimum, and stability (for a review, see Creissen et al., 1994). The APx protein has been purified from pea, spinach, tea, and maize (Zea mays) (Chen and Asada, 1989; Mittler and Zilinskas, 1991a; Tanaka et al., 1991; Koshiba, 1993). Chloroplastic APx isoforms have been well characterized as scavengers of photoproduced hydrogen peroxide in the chloroplasts. The role of cytosolic APx in the cell still remains to be clarified. cDNA and genomic clones encoding cytosolic APx have been isolated from soybean root nodules (Chatfield and Dalton, 1993), pea (Mittler and Zilinskas, 1991b, 1992), and Arabidopsis thaliana (Kubo et al., 1992; S. Kushnir, H. Willekens, G. Bauw, M. Van Montagu, and D. Inze, unpublished data). In maize, the N-terminal amino acid sequence of a purified APx showed high similarity to pea, spinach, and Arabidopsis cytosolic APx but not to chloroplastic APx from tea (Koshiba, 1993). We report the isolation and sequencing of a maize cDNA clone coding for a cytosolic APx (Table I). A maize seedling AZAP library was screened with an Arabidopsis APx probe. Putative APx cDNA clones were isolated at a frequency of 1:40,000. A clone with an insert of 1.05 kb was purified until homogeneity and sequenced. Sequence analysis of both strands resulted in a 1042-bp nucleotide sequence with a 3’ poly(A) tail and an open reading frame encoding 250 amino acids. Northern analysis showed an apx transcript of approximately 1 kb in 14-d-old seedlings, indicating that the isolated APx cDNA is full length. The coding region of the maize cDNA showed 73% similarity with the APx cDNAs isolated from pea and A. thaliana (Mittler and Zilinskas, 1991b; Kubo et