Photosynthetic electron carriers are important in converting light energy into chemical energy in green plants. Although protein components in the electron transport chain are largely conserved among plants, algae and prokaryotes, there is thought to be a major difference concerning a soluble protein in the thylakoid lumen. In cyanobacteria and eukaryotic algae, both plastocyanin and cytochrome c(6) mediate electron transfer from cytochrome b(6)f complex to photosystem I. In contrast, only plastocyanin has been found to play the same role in higher plants. It is widely accepted that cytochrome c(6) has been evolutionarily eliminated from higher-plant chloroplasts. Here we report characterization of a cytochrome c(6)-like protein from Arabidopsis (referred to as Atc6). Atc6 is a functional cytochrome c localized in the thylakoid lumen. Electron transport reconstruction assay showed that Atc6 replaced plastocyanin in the photosynthetic electron transport process. Genetic analysis demonstrated that neither plastocyanin nor Atc6 was absolutely essential for Arabidopsis growth and development. However, plants lacking both plastocyanin and Atc6 did not survive.