Tryptophan gene-enzyme relationship

There is currently some uncertainty about the complexity of the tryptophan gene-enzyme relationships in B. subtilis. Whitt and Carlton [46,47] have noted pleiotropic effects. Most recently [46] they have found that the pleiotropy is limited to elimination of InGPS activity by mutations in either the trpD or trpF genes, which are primarily concerned with PRT and PRAI activity, respectively. The pleiotropic effects are shown in Fig. 3 by dotted lines and may indicate that the enzymes function as aggregates in vivo or may represent effects on translation similar to polarity effects. Hoch et al. [48], on the other hand, have not found these pleiotropic effects and report essentially one gene-one enzyme activity [when the individual activities of the tryptophan synthetase a and p2 subunits (Fig. 1) are included]. The nonconformity in the results of different investigators may be due to the use of different mutants and to different methods of preparation, affecting enzyme stabilities. 

Niacin requirements are dependent on the tryptophane supply and the ease with which the conversion of tryptophane to niacin can be made. Chickens and rats carry out the conversion readily. Dogs do so less readily. Monkeys and human beings carry out the process relatively ineffectively. Since this conversion involves several enzymatic steps, it is clear on the basis of gene-enzyme relationships why species differences exist. On the same basis inter-individual differences may be presumed to exist also. 

Fig. 7. Intermediates and gene-enzyme relationships of tryptophan biosynthesis in Neurospora. Abbreviations PRPP, 5-phosphoribosyl-l-pyrophosphate PRA, N-(5 -phosphoribosyl) anthranilic acid CDRP, l-(o-carboxyphenylamino)-l-deoxyribulose-5-phosphate InGP, indole-3-glycerolphosphate PR, phosphoribosyl.

III. Gene-Enzyme Relationships in the Tryptophan Biosynthetic Pathway. 392... 

III. GENE-ENZYME RELATIONSHIPS IN THE TRYPTOPHAN BIOSYNTHETIC PATHWAY... 

Fio. 3. Gene-enzyme relationships in the tryptophan pathway of bacteria. The gene designated by a question mark represents a possible location (inferred from mapping distances) of the gene coding for the smaller protein component of AS in P. putida. The gene has not yet been identified by a specific mutational defect. Dashed lines represent relationships whose existence are still in question. See the text for further explanations. 

Tentative gene-enzyme relationships in the tryptophan biosynthetic pathway of B. subtilis are shown in Fig. 3. Early studies indicating that... 

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