It is thus obvious that the synthesis of tryptophan is self regulatory, since tryptophan, when present in plenty, works firstly by feed back inhibition, and secondly through co-repression to stop further synthesis of tryptophan. The major difference between lactose operon and tryptophan operon is that tryptophan increases the affinity of repressor protein for the operator. In tryptophan operon there is yet another mechanism for regulation of tryptophan synthesis through leader sequences and attenuation, which is discussed later in this section.
Tryptophan (trp) repressor controls three sets of genes
The lac repressor acts only on lac operon with ZYA cluster of genes. However, a repressor can control several groups of genes dispersed in the genome.
For instance, trp repressor controls following three sets of unlinked genes : (i) trp EDBCA meant for tryptophan synthesis enzymes; (ii) aroH gene coding for an enzyme needed for common pathway of aromatic amino acid biosynthesis, and (iii) trp R regulator gene (repressed by its own gene product = autogenous control). A related operator sequence (27bp long) is present at each of these three genes/ gene clusters, although the location of this sequence relative to start point may differ (as shown earlier). The operator may actually lie within the promoter (e.g. trpP locus), or downstream (e.g. lac)or upstream (e.g. gal).
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