A SPONTANEOUS POINT MUTATION IN THE RUBISCO LARGE SUBUNIT GENE IMPAIRING HOLOENZYME ASSEMBLY RENDERS THE IV-BETA PLASTOME MUTANT OF OENORHERA EXTREMELY LIGHT-SENSITIVE AND CHILLING SENSITIVE

A spontaneously occurring chloroplast genome (plastome) mutant of Oeno thera, IV beta, was identified as a single point mutation in the Rubis co large subunit gene (G337 --> C), leading to an V113L exchange, whic h 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 accu mulation of a processing precursor of the holoenzyme, the B-complex, c onsisting of one LSU and 14 units of chaperonine 60 (cpn60). It is ass ociated with very low holoenzyme concentrations in the mutant tissue, but does not affect the kinetic properties of the enzyme once assemble d. When grown under moderate or low light, leaf tissue containing the plastome mutant showed decreased Chi contents and Chi ab ratios, incre ased relative carotenoid contents and violaxanthin deepoxidation activ ity, but very low CO2 fixation and O-2 evolution rates and was very se nsitive to photoinhibition. The light-dependence of chlorophyll fluore scence quenching components at low temperature resembled an extremely chilling sensitive Oenothera genotype as compared to the wild-type. Th e IV beta mutant thus behaves similarly to the Rubisco SSU antisense p lants analysed by Stitt and co-workcrs (summarised by Stitt and Schulz e 1994) and gives an example of the possible influence of plastome mut ations on the sensitivity of the photosynthetic apparatus to excess li ght by modifying the capacity of the Calvin cycle.