title: Nuclear mutation restores the reduced CO2/O2 specificity of ribulosebisphosphate carboxylase/oxygenase in a temperature-conditional chloroplast mutant of Chlamydomonas reinhardtii.
abstract: The Chlamydomonas reinhardtii temperature-sensitive mutant 68-4PP results from a mutation within the chloroplast gene that encodes the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. When grown at the permissive temperature (25 degrees C), the mutant has a reduced level of holoenzyme protein, and the purified enzyme has a lower CO2/O2 specificity than the wild-type enzyme. At the nonpermissive temperature (35 degrees C), the holoenzyme level is greatly reduced, and the mutant is unable to grow photosynthetically. When photosynthesis-competent revertants of 68-4PP were selected at 35 degrees C, a nuclear mutation was identified that suppresses the temperature-sensitive phenotype by enhancing both the activity and amount of the mutant enzyme. More significantly, the reduced CO2/O2 specificity of the 68-4PP enzyme is restored to the wild-type value. However, the nuclear suppressor mutation alone does not produce a phenotype different from wild type, and the CO2/O2 specificity of the suppressor strain's enzyme is normal. We have cloned and completely sequenced the two small-subunit genes from the suppressor strain, but no mutation has been found. These results suggest that some other nuclear-encoded protein is able to influence the structure of the holoenzyme, which in turn influences the CO2/O2 specificity factor.
authors.affiliation: Department of Biochemistry, University of Nebraska, Lincoln 68583-0718.
authors.forename: Z X
authors.initials: ZX
authors.lastname: Chen
authors.forename: D
authors.initials: D
authors.lastname: Green
authors.forename: C
authors.initials: C
authors.lastname: Westhoff
authors.forename: R J
authors.initials: RJ
authors.lastname: Spreitzer
mesh_terms: D001483:Base Sequence; D002245:Carbon Dioxide / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism*; D002696:Chlamydomonas / Q000201:enzymology / Q000235:genetics*; D002736:Chloroplasts / Q000201:enzymology*; D003001:Cloning, Molecular; D003062:Codon / Q000235:genetics; D003433:Crosses, Genetic; D007700:Kinetics; D008969:Molecular Sequence Data; D009154:Mutation; D015345:Oligonucleotide Probes; D010100:Oxygen / Q000378:metabolism; D010641:Phenotype; D011544:Pseudogenes; D015183:Restriction Mapping; D012273:Ribulose-Bisphosphate Carboxylase / Q000235:genetics* / Q000378:metabolism; D013379:Substrate Specificity; D013696:Temperature
chemical_list: D003062:Codon; D015345:Oligonucleotide Probes; D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase; D010100:Oxygen