Effects of selective inactivation of individual genes for low-molecular-mass subunits on the assembly of photosystem II, as revealed by chloroplast transformation: The psbEFLJ operon in Nicotiana tabacum

Photosystem (PSII) is a supramolecular polypeptide complex found in oxygenic photosynthetic membranes, which is capable of extracting electrons from water for the reduction of plastoquinone. An intriguing feature of this assembly is the fact that it includes more than a dozen low-mass polypeptides of generally unknown function. Using a transplastomic approach, we have individually disrupted the genes of the psbEFLJ operon in Nicotiana tabacum, which encode four such polypeptides, without impairing expression of downstream loci of the operon. All four mutants exhibited distinct phenotypes; none of them was capable of photoautotrophic growth. All mutants bleached rapidly in the light. Disruption of psbE and psbF, which code for the α and β apoproteins of cytochrome b₅₅₉, abolished PSII activity, as expected; ΔpsbL and ΔpsbJ plants displayed residual PSII activity in young leaves. Controlled partial solubilisation of thylakoid membranes uncovered surprisingly severe impairment of PSII structure, with subunit and assembly patterns varying depending on the mutant considered. In the ΔpsbL mutant PSII was assembled primarily in a monomeric form, the homodimeric form was preponderant in ΔpsbJ, and, unlike the case in ΔpsbZ, the thylakoids of both mutants released some PSII super-complexes. On the other hand, Photosystem I (PSI), the cytochrome b₆f complex, ATP synthase, LHCII, and CP24/CP26/CP29 antennae were present in near wild-type levels. The data are discussed in terms of their implications for structural, biogenetic and functional aspects of PSII.