The genetics of chemical diversity.

The plethora of natural organic chemicals contrasts with the relative scarcity of genes and the apparent difficulty to evolve new ones. The genetical analysis of metabolism may be reviewed with this paradox in mind. The terpenoids constitute a particularly varied group of natural compounds; many of them are dispensable to the cell and their biosynthesis is amenable to mutational analysis and other genetical and chemical methods. The production of carotene and gibberellins by the fungi Phycomyces blakesleeanus and Gibberella fujikuroi, respectively, seems to require an unexpectedly small number of genes. A number of gene-saving devices are detected that may have general validity for other cases of secondary metabolism. The most important one is versatile genes whose products are specific for a chemical reaction but not for the substrate. This versatility allows a combinatorial use that increases chemical and behavioral diversity. Physical separation of cellular functions in compartments or enzyme aggregates not only makes processes more efficient but helps avoid some deleterious consequences of enzyme versatility.