Nuclear genes required for post-translational steps in the biogenesis of the chloroplast cytochrome b6f complex in...

Nuclear genes essential for the biogenesis of the chloroplast cytochrome b6f complex were identified by mutations that cause the specific loss of the complex. We describe four transposon-induced maize mutants that lack cytochrome b6f proteins but contain normal levels of other photosynthetic complexes. The four mutations define two nuclear genes. To identify the step at which each mutation blocks protein accumulation, mRNAs encoding each subunit were examined by Northern hybridization analysis and the rates of subunit synthesis were examined in pulse-labeling experiments. In each mutant the mRNAs encoding the known subunits of the complex were normal in size and abundance and the major subunits were synthesized at normal rates. Thus, these mutations block the biogenesis of the cytochrome b6f complex at a post-translational step. The two nuclear genes identified by these mutations may encode previously unknown subunits, be involved in prosthetic group synthesis or attachment, or facilitate assembly of the complex. These mutations were also used to provide evidence for the authenticity of a proposed fifth subunit of the complex and to demonstrate a role for the cytochrome b6f complex in protecting photosystem II from light-induced degradation.