SOMERVILLE, CHRIS. Carnegie Institution and Department of Biological Sciences, Stanford University, Stanford CA 94305. - The evolution of chemical diversity.
The evolution of higher plants has been accompanied by the evolution
of enzymes that lead to the production of a wide range of chemical
constituents. Chemical structures are known for more than 15,000
chemical constituents on plants and, by extrapolation, it has been
estimated that higher plants collective produce more than 100,000
chemical constituents. A relatively simple class of compounds that
exhibit substantial chemical diversity is fatty acids that accumulate
as major constituents of storage oils in many species. Many of the
genes encoding the enzymes required for fatty acid desaturation were
cloned by molecular genetic techniques in Arabidopsis. These
genes were then used to identify variants of the genes from diverse
plant species. These studies revealed that essentially all chemical
diversity in plant fatty acids was due to the action of structural
variants of a single enzyme. Mechanistic studies of this enzyme family
showed that minor changes in the amino acid composition of the protein
near the active site could change the outcome of the reaction
catalyzed by this family of enzymes. In one instance we were able to
show that as few as four amino acid changes in an enzyme could convert
and enzyme that normally inserted a double bond into fatty acids into
an enzyme that inserted a hydroxyl group. These studies provide an
explanation for the mechanism by which chemical diversity in this
class of compounds has evolved and provide a model for understanding
the mechanisms underlying the evolution of other major families of
compounds.
Key words: Arabidopsis, chemical diversity, evolution, genomics, proteomics