A Mutant of <i>Arabidopsis thaliana</i> Which Lacks Activation of RuBP Carboxylase <i>In Vivo</i>

A mutant of Arabidopsis thaliana has been isolated in which ribulose-1,5-bisphosphate carboxylase is present in a nonactivatable form in vivo. The mutation appears to affect carboxylase activation specifically, and not any other enzyme of the photosynthesis or photorespiratory cycles. The effect of the mutation on carboxylase activation is indirect, inasmuch as the properties of ribulose-1,5-bisphosphate carboxylase purified from the mutant are not distinguishable from those of the wild type enzyme. The mutant requires high levels of atmospheric CO(2) for growth because photosynthesis is severely impaired in atmospheres containing normal levels of CO(2), irrespective of the atmospheric O(2) concentration. In this respect, the mutant is distinguished from previously described high-CO(2) requiring mutants of Arabidopsis which have defects in photorespiratory carbon or nitrogen metabolism.