Component of the cytochrome b6-f complex, which mediates electron transfer between photosystem II (PSII) and photosystem I (PSI), cyclic electron flow around PSI, and state transitions. PetL is important for photoautotrophic growth as well as for electron transfer efficiency and stability of the cytochrome b6-f complex. The 4 large subunits of the cytochrome b6-f complex are cytochrome b6, subunit IV (17 kDa polypeptide, PetD), cytochrome f and the Rieske protein, while the 4 small subunits are PetG, PetL, PetM and PetN. The complex functions as a dimer. The N-terminus is blocked. A deletion of this gene can be substituted for by a subunit IV-petL fusion. The fusion protein does not permit state transitions, although all other functions are wild-type. Belongs to the PetL family. Mass spectrometry indicates that the start site is probably Met-1, and not Met-12 as is thought to be the case in other organisms.