Convergence, stability, and thermal adaptation in the rubisco enzyme in plants

Enzymes are thought to be tuned to perform similarly in different thermal regimes. Whether the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) follows similar rules, especially when considering evolutionary history, is uncertain. The molecular, structural, and ecological factors of the rubisco large subunit (RbcL) were examined in four plant clades: wood ferns, pines, sea lavenders, and viburnums. Using rbcL gene sequences, codon evolutionary models were used to test for positive and divergent selection and convergent evolution. Protein structure modeling was performed to predict side chain changes and protein stability. Phylogenetic comparative methods were used to examine the relationship between protein stability to growing season temperature. All four clades showed significant evidence of positive selection, with multiple convergent substitutions predicted to alter side chain polarity and interactions with the solvent. In viburnums, biome transition rates were dependent on amino acid substitution, with positive selection was concentrated in cold temperate and cloud forest clades. Rubiscos with higher stability occurred in species from warmer environments. However, this correlation was weaker after correcting for phylogeny. These analyses support a hypothesis that RbcL evolution is influenced by both environmental tuning and evolutionary history.