A Mutant Strain of Chlamydomonas reinhardi Exhibiting Altered Ribulosebisphosphate Carboxylase

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Abstract
A mutant, ac i72, of Chlamydomonas reinhardi possessing an altered ribulosebisphosphate carboxylase and unable to grow on minimal medium has been isolated and characterized. Comparison of ribulosebisphosphate carboxylase purified from both wild type and ac i72 strains is given. The enzyme from ac i72 shows alterations in several characteristics: (a) the specific activity is reduced to 35% that of wild type, (b) the V for both substrates is reduced 3-6 fold, (c) the Mg2+ requirement for maximal activity is 3 times greater, (d) the inhibitory effect of Cl- is greater, and (e) the isoelectric point is changed (6.0 for wild type and 5.8 for ac i72). However, the ribulosebisphosphate carboxylase from ac i72 is identical to that from wild type with respect to pH requirement, temperature sensitivity, subunit structure, and sedimentation characteristic. Other photosynthetic properties of wild type and ac i72 cells were also compared. CO2 fixation in ac i72 in vivo is reduced proportionally to the reduction in activity of the enzyme, but the level of O2 evolution is the same as in wild-type cells. Photosynthetic electron transport, 70-S ribosome content, and chlorophyll content are unaltered in ac i72. The chloroplast ultrastructure of ac i72 cells is distinctly different from that of wild-type cells. The inheritance of the mutation is Mendelian.