Deletion of nine carboxy-terminal residues of the Rubisco small subunit decreases thermal stability but does not eliminate function

A recent X-ray crystal structure of ribulose-1,5-bisphosphate carboxylase/oxygenase from the green alga Chlamydomonas reinhardtii lacks 13 carboxy-terminal residues of the small subunit. To determine the importance of this divergent region, a non-sense mutation was created that removes nine residues. This engineered gene was transformed into a Chlamydomonas strain that lacks the small-subunit gene family. The resulting holoenzyme has a normal CO₂/O₂ specificity but decreased carboxylation V_max. Whereas wild-type enzyme retained most of its carboxylase activity after a 10-min incubation at 55°C, the mutant enzyme was inactivated. Thus, although disordered or divergent, the carboxy terminus is required for maximal activity and stability.