Evolution
It is thought that early in evolution the trpB2 gene was duplicated. One copy entered the trp operon as trpB2i allowing for its expression with trpA. TrpB2i formed transient complexes with TrpA and in the process activated TrpA unidirectionally. The other copy remained outside as trpB2o, and fulfilled an existing role or played a new one such as acting as a salvage protein for indole. TrpB2i evolved into TrpB1, which formed permanent complexes with trpA resulting in bidirectional activation. The advantage of the indole salvage protein declined and the TrpB gene was lost. Finally, the TrpB1 and TrpA genes were fused resulting in the formation the bifunctional enzyme.
Read more about this topic: Tryptophan Synthase
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