gamma-Glutamylcysteine synthetase (GCS) is the initial and rate-limiting enzyme in the glutathione (GSH) de novo synthesis pathway. GCS is composed of a heavy (73-kDa) catalytic subunit and a light (30-kDa) regulatory subunit, which maintains the Km for glutamate near physiologic concentrations. Previous studies have shown that the steady-state mRNA level and gene transcription for the catalytic subunit increased in response to the redox-cycling quinone 2,3-dimethoxy-1,4-naphthoquinone (DMNQ) in rat lung epithelial L2 cells (M. M. Shi, et al., 1994, J. Biol. Chem. 269,26512-26517). The ratio of the catalytic to regulatory subunit mRNAs varies among tissues, and the anticancer drug cisplatin appears to induce only the catalytic subunit, suggesting independent gene regulation of the two subunits. Nonetheless, the present study found that the steady-state mRNA level and the transcription rate of the GCS regulatory subunit also increased under DMNQ-induced oxidative stress. Changes in mRNA followed a pattern similar to that for the catalytic subunit. The mRNA levels of the two subunits of GCS also both increased above the baseline levels in cells treated with BSO, an inhibitor of GCS enzymatic activity. These data suggest that, under conditions of oxidative stress or glutathione depletion, the regulatory subunit is upregulated at the level of mRNA transcription. Along with the elevation of the catalytic subunit, this increase in GCS regulatory subunit transcription contributes to increases in GCS enzymatic activity and cellular GSH content.