Evolutionary divergence in the two kinds of subunits of ribulose diphosphate carboxylase isolated from different species ofNicotiana

A ² analysis was made of previously reported values for the amino acid composition of the large and small subunits of ribulose diphosphate carboxylase (E.C. 4, 1, 1, 39) isolated from five species ofNicotiana. The distributions of these values were then compared with published values for hemoglobin chains and cytochromec's of diverse origins. It was concluded that evolutionary diversity in the large and small subunits of RuDP carboxylase occurs even within the limited taxonomic category of the genusNicotiana. The large subunit was as stable towards mutation during evolution as the chains and cytochromes, whereas the small subunit was much less stable. The hypothesis is discussed that the large subunit played a more significant role than the small subunit in the enzyme function and/or structural integrity of the oligomeric protein. It was also speculated that the addition of small subunits to the molecule, which may have been a recent evolutionary event, enables the fixation of many evolutionarily favorable mutations. Thus, survival of the enzymatic activity in changing environments is favored.