Tryptophan biosynthetic genes in eukaryotic microorganisms.

In recent years more information about tryptophan biosynthesis in eukaryotic microorganisms has become available. The emphasis has been on genetics and biochemistry of the pathway. Eukaryotes manifest a trend toward fewer genes and toward multifunctional proteins, while prokaryotes have a greater tendency toward separate activity domains but the genes tend to be clustered genetically. Cloning of various structural tryptophan biosynthetic genes and studies on their expression in homologous and heterologous hosts have made it possible to analyze promoter structures in detail and to define structural elements involved in regulated gene expression. Comparisons of homologous genes from different organisms have highlighted the conservation of the activity domains or parts therefrom involved in the catalysis of single steps. These studies also point to a stringent maintenance of domains responsible for protein-protein aggregation. Physiological studies will be facilitated by the availability of single cloned genes and especially the artificial gene cluster containing all five TRP genes from yeast. The range of physiological manipulation has thus been enormously broadened. With chromosomal mutations it has been possible to study primarily downward modulation of a pathway. We can now initiate studies on upward modulation, since enzyme levels appear to increase in proportion to gene dose. The new range of downward and upward modulation in the levels of single enzymes and combinations of enzymes may contribute to a better understanding of flux regulation and its influence on the overall physiology of an organism.