During photoinhibition of photosystem II (PSII) in cyanobacteria, salt stress inhibits the repair of photodamaged PSII and, in particular, the synthesis of the D1 protein (D1). We investigated the effects of salt stress on the repair of PSII and the synthesis of D1 in wild-type tobacco (Nicotiana tabacum 'Xanthi') and in transformed plants that harbored the katE gene for catalase from Escherichia coli. Salt stress due to NaCl enhanced the photoinhibition of PSII in leaf discs from both wild-type and katE-transformed plants, but the effect of salt stress was less significant in the transformed plants than in wild-type plants. In the presence of lincomycin, which inhibits protein synthesis in chloroplasts, the activity of PSII decreased rapidly and at similar rates in both types of leaf disc during photoinhibition, and the observation suggests that repair of PSII was protected by the transgene-coded enzyme. Incorporation of [(35)S]methionine into D1 during photoinhibition was inhibited by salt stress, and the transformation mitigated this inhibitory effect. Northern blotting revealed that the level of psbA transcripts was not significantly affected by salt stress or by the transformation. Our results suggest that salt stress enhanced photoinhibition by inhibiting repair of PSII and that the katE transgene increased the resistance of the chloroplast's translational machinery to salt stress by scavenging hydrogen peroxide.