A spontaneous point mutation in the Rubisco large subunit gene impairing holoenzyme assembly renders the IVβ plastome mutant of Oenorhera extremely light- and chilling sensitive

A spontaneously occurring chloroplast genome (plastome) mutant of Oenothera, IV beta, was identified as a single point mutation in the Rubisco large subunit gene (G337 --> C), leading to an V113L exchange, which topologically occurs at the interface of two adjacent large subunits (LSU). The minor sterical hindrance of dimer formation by this amino acid exchange strongly impairs holoenzyme assembly, leading to an accumulation of a processing precursor of the holoenzyme, the B-complex, consisting of one LSU and 14 units of chaperonine 60 (cpn60). It is associated with very low holoenzyme concentrations in the mutant tissue, but does not affect the kinetic properties of the enzyme once assembled. When grown under moderate or low light, leaf tissue containing the plastome mutant showed decreased Chi contents and Chi ab ratios, increased relative carotenoid contents and violaxanthin deepoxidation activity, but very low CO2 fixation and O-2 evolution rates and was very sensitive to photoinhibition. The light-dependence of chlorophyll fluorescence quenching components at low temperature resembled an extremely chilling sensitive Oenothera genotype as compared to the wild-type. The IV beta mutant thus behaves similarly to the Rubisco SSU antisense plants analysed by Stitt and co-workcrs (summarised by Stitt and Schulze 1994) and gives an example of the possible influence of plastome mutations on the sensitivity of the photosynthetic apparatus to excess light by modifying the capacity of the Calvin cycle.