THE STRUCTURE AND FUNCTION OF RUBISCO AND THEIR IMPLICATIONS FOR SYSTEMATIC STUDIES

As ''the most abundant protein in the world,'' ribulose-1,5-bisphospha te carboxylase (RuBisCO) attracts the attention df genetic engineers a nd plant phylogeneticists. The active site, which is responsible for a lmost all carbon fixation bn earth, is in the large subunit (LSU). Ove r 30% of the 476 amino acids in the LSU are involved in intermolecular associations. Using available sequence data, we find that 105 (22%) o f the residues are absolutely conserved across 499 seed plants, with a n 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 numb er of potentially variable sites is likely to be slightly over 1000, r ather than 1428. Second, rates of change can vary greatly across the m olecule; functional constraints on amino acids and codon biases greatl y increase the potential for homoplasy. Third, some changes are correl ated, and thus might be down-weighted accordingly. Fourth, some of the variation in RuBisCO may be adaptive and present insights into the na ture of evolutionary change in response to the environment.