SummaryThe psae1‐1 mutant of Arabidopsis was identified on the basis of a decrease in the effective quantum yield of photosystem II, among a collection of plants subjected to transposon tagging with the Enhancer element. The steady‐state redox level and the rate of re‐oxidation of P700 are significantly altered in psae1‐1 mutants. The responsible mutation was localised to psaE1, one of two Arabidopsis genes that encode subunit E of photosystem I. An additional mutant allele, psae1‐2, was identified by reverse genetics. In wild‐type plants, the psaE1 transcript is expressed at a higher level than psaE2 mRNA. In the mutants, however, the E1 transcript was barely detectable, and was expressed only in small groups of wild‐type cells resulting from somatic reversions. As a consequence, the amount of PsaE protein present in the mutant is significantly reduced. Concomitantly, the levels of other stromal photosystem I subunits (PsaC and PsaD) are also affected. Mutant plants showed a marked increase in light sensitivity and photoinhibition. Additional effects of the psae1 mutation include light green pigmentation, an increase in chlorophyll fluorescence and a decrease of approximately 50% in growth rate under greenhouse conditions.