LAUSR

The evolution of ribulose-1,5-bisphosphate carboxylase/oxygenases (Rubiscos) that discriminate strongly between their substrate carbon dioxide and the undesired side substrate dioxygen was an important event for photosynthetic organisms adapting to an oxygenated environment. We use ancestral sequence reconstruction to recapitulate this event. We show that Rubisco increased its specificity and carboxylation efficiency through the gain of an accessory subunit before atmospheric oxygen was present. Using structural and biochemical approaches, we retrace how this subunit was gained and became essential. Our work illuminates the emergence of an adaptation to rising ambient oxygen levels, provides a template for investigating the function of interactions that have remained elusive because of their essentiality, and sheds light on the determinants of specificity in Rubisco. Description Origins of Rubisco’s small subunit sidekick Maintaining the correct activity, specificity, and stability is a challenge faced by metabolic enzymes that is often solved by forming multimeric complexes. The large, catalytic subunit of form I Rubisco, the carbon-fixing enzyme found in aerobic photosynthetic organisms such as plants and cyanobacteria and in some nonphotosynthetic bacteria, makes such a complex with an essential small subunit. Using ancestral sequence reconstruction, Schulz et al. investigated how recruiting this small subunit contributed to Rubisco evolution and, in particular, its specificity for carbon dioxide (see the Perspective by Sharwood). Enzyme activity assays and macromolecular structures revealed that the small subunit quickly became essential and opened the door to broader functional changes within the large subunit, including increased activity and specificity for carbon dioxide. —MAF Ancestral sequence reconstruction reveals how recruiting a new subunit primes form I Rubiscos for rising atmospheric oxygen.