Synthesis of tryptophan in bacteria and Neurospora
These mutants indicate that, in the first case : (i) mutants are incapable of utilizing ornithine or citrulline; in the second case, (ii) mutants are incapable of utilizing ornithine; but in the last case, (iii) only the precursor cannot be utilized. The biosynthetic pathway, suggested on the basis of these mutation studies, is shown in Figure 22.8.
|Fig. 22.8. Different steps in the synthesis of arginine and three different mutations blocking three steps.
In Salmonella typhimurium,
four kinds of mutants for tryptophan synthesis could be isolated. These mutations are listed in Table 22.2, and include the following : (i) trp-8
does not have the ability to synthesize anthranilic acid but the growth can be restored by addition of either of the following compounds—anthranilic acid, indole glycerol phosphate, indole or tryptophan. (ii) trp-2
can not grow on either the minimal medium or on the supplemented medium with anthranilic acid, but can grow on addition of either indole glycerol phosphate, or indole or tryptophan. (iii) trp-3
can grow only on a supplemented medium with either indole or tryptophan, but not on any other medium, (iv) trp-1, 6, 7, 9, 10, 11,
etc. can grow only on a supplemented medium with tryptophan and on no other medium. These different mutants indicate interruption of biochemical pathway at different steps.
In Escherichia coli
also five different kinds of mutations called trp A, trp B, trp C, trp D
and trp E
were identified. These mutations are similar to those described for Salmonella,
and are shown in Figure 22.9 and are summarized in Table 22.2. The biosynthetic pathway shown in Figure 22.9 was traced out using the mutants described above for Salmonella typhimurium
and Escherichia coli.
Tryptophan synthetase in Neurospora and bacteria.
|Fig. 22.9. Different steps in the synthesis of amino acid tryptophan, showing the enzymes and genes for each step.
It has been shown that the last two steps of tryptophan metabolism in Neurospora
as well as in bacteria are controlled by the same enzyme tryptophan synthetase,
so that even mutants lacking the ability to utilize indole do not have tryptophan synthetase
enzyme. In Neurospora,
this enzyme consists of only one component. But in E. coli
it has two components : (i) 'A' component affecting penultimate step in Figure 22.9, so that trp A
mutants lacking A component can be supplemented by both indole and tryptophan and (ii) 'B' component, which affects last step in Figure 22.9, so that trpB
mutants lacking B component must be supplemented directly with tryptophan. It has also been shown that in mutants for this enzyme, protein is synthesized, but it is unable to perform enzymatic function.
The presence of two components (A and B) of tryptophan synthetase enzyme, synthesized under the control of two different genes suggests that a particular enzyme may consist of more than one polypeptides, each synthesized under the control of a different functional unit called cistron
(see Fine Structure of Gene-at the Genetic Level (A New Concept of Allelomorphism)
). Therefore, it was suggested that the concept of one gene-one enzyme
proposed initially by Beadle
should be modified into one cistron-one polypeptide hypothesis.