Chloroplast intragenic suppression enhances the low CO2/O2 specificity of mutant ribulose-bisphosphate carboxylase/oxygenase.

The competition between CO2 and O2 at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase limits net CO2 fixation in photosynthesis. In the green alga Chlamydomonas reinhardtii, a mutation in the chloroplast large-subunit gene reduces the CO2/O2 specificity of the enzyme by 37% and causes valine-331 to be replaced by alanine. Revertant selection identified an intragenic suppressor mutation that increases the CO2/O2 specificity of the mutant enzyme by 33%. This second-site mutation causes threonine-342 to be replaced by isoleucine. The complementing amino acid substitutions flank a catalytically essential lysyl residue at position 334. It thus appears that a number of amino acid residues can influence the CO2/O2 specificity of this bifunctional enzyme. The well defined chloroplast genetics of C. reinhardtii allows the interactions of these residues to be investigated.