A cDNA clone encoding the 27-kilodalton subunits of glutathione S-transferase IV from Zea mays.

The GST (EC 2.5.1.18) family of enzymes are well known for their role in the detoxification of various xenobiotic compounds in both plant and animal systems. These enzymes facilitate the addition of GSH, y-glutamylcysteinylglycine, to an electrophilic site of a suitable substrate, yielding a GSH conjugate. In maize (Zea mays), the GSH-conjugation reaction is responsible for the metabolism of severa1 classes of pesticides to nonphytotoxic forms (Lamoureux and Rusness, 1989). Multiple GSTs have been identified in maize and include GST isozymes I, 11,111, and IV (Timmerman, 1989; Irzyk and Fuerst, 1993). These isozymes differ with respect to molecular mass, subunit composition (heteroor homodimeric), and substrate specificity. Nucleotide sequences and deduced amino acid sequences have previously been reported for maize GSTs I, 11, and I11 (Moore et al., 1986; Wiegand et al., 1986; Grove et al., 1988; Bridges et al., 1993). Herbicide safeners are compounds used to protect crops, such as maize, from herbicide injury. In maize, both GST activity and herbicide metabolism, via GSH conjugation, are stimulated by herbicide safener treatment (Viger et al., 1991). We have previously identified and purified a GST isozyme from maize that is induced by treatment with the herbicide safener benoxacor [4-(dichloroacetyl)-3,4dihydro-3-methyl-2H-1,4-benzoxazine; Peek et al., 19881 and designated this isozyme as maize GST IV (Irzyk and Fuerst, 1993). Native GST IV is a homodimer composed of two 27-kD subunits and can conjugate GSH to choloracetamide and S-triazine substrates but not to l-chloro-2,4dinitrobenzene, a model GST substrate. A full-length cDNA clone, GST27 (Table I), was isolated from a maize shoot cDNA library constructed using the A ZAP I1 vector (Stratagene). The library was screened with an approximately 450-bp PCR product that was amplified from maize first-strand cDNA using degenerate primers designed from partia1 amino acid sequence of the purified GST IV protein (Irzyk and Fuerst, 1993). The PCR primers