The structure and function of RuBisCO and their implications for systematic studies.

As "the most abundant protein in the world,'' ribulose-1,5-bisphosphate carboxylase (RuBisCO) attracts the attention of genetic engineers and plant phylogeneticists. The active site, which is responsible for almost all carbon fixation on earth, is in the large subunit (LSU). Over 30% of the 476 amino acids in the LSU are involved in intermolecular associations. Using available sequence data, we find that 105 (22%) of the residues are absolutely conserved across 499 seed plants, with an additional 110 demonstrating only one change. Our analyses show that conserved domains are not fully explained by current structural data. This has several implications for systematic studies. First, the number of potentially variable sites is likely to be slightly over 1000, rather than 1428. Second, rates of change can vary greatly across the molecule; functional constraints on amino acids and codon biases greatly increase the potential for homoplasy. Third, some changes are correlated, and thus might be down-weighted accordingly. Fourth, some of the variation in RuBisCO may be adaptive and present insights into the nature of evolutionary change in response to the environment.