Histidine metabolism in Salmonella
In this section, earlier we have shown that each step of a biosynthetic pathway is controlled by a specific enzyme synthesized under the influence of a specific gene. In several cases, although not universally, it has been shown that the genes involved in different steps of a biosynthetic pathway are clustered and found in the same sequence in which their corresponding enzymes are used in the biosynthetic pathway. This will be illustrated with the help of two examples.
Tryptophan metabolism in Salmonella
It has been earlier shown that for tryptophan synthesis in Salmonella, five enzymes are needed for five steps earlier described as 1, 2, 3, 4, 5. It was found that the genes for three enzymes in Salmonella are found in the same order in which the steps are found in the biosynthetic pathway.
More than 500 mutations for histidine metabolism in Salmonella
were mapped. These were linked in nine closely linked loci A to H, each affecting a different enzyme in the synthesis of histidine. The gene sequence was found to follow largely the
For several other traits including proline, threonine and isoleucine - valine metabolism in Salmonella,
a similar clustering of genes was observed. This suggests, that at a primitive stage biochemical synthesis during evolution must have first organized on the chromosome itself.