The Tryptophan Operon and its Regulation
The Tryptophan (trp) Operon is an operon – a set of genes that are transcribed, translated, and controlled as a group – that generates the enzymes and protein structures necessary for the biosynthesis of tryptophan amino acid molecules by Prokaryotic organisms. In 1953, Jacques Monod and his colleagues discovered the trp operon in E. coli as the first repressible operon to be known. Since then, the trp operon has been a commonly used example of a repressible system operated by negative control. This type of control indicates that the operon is naturally active and must have a repressible structure to bind and inhibit activity and production. This operon is both activated and repressed based on the levels of effector molecules – tryptophan – in the environment by means of regulation and attenuation.
Brief Evolutionary Descent
The tryptophan (trp) operon is considered an ancient innovation for it was present in bacterial and archaeal ancestors prior to their branching. The first evolutionary steps were gathering and organizing structural genes therefore at first regulation was very minimal. According to parisomony principles, there were two milestone evolutionary events, one splitting the operon in two and one rejoining it by gene fusion, both mapped to the 16S rRNA tree of bacteria. Although lateral gene transfer can be asserted, Gogarten recently conducted a “synthesis” illustrating vertical decent of genes as well as a horizontal gene transfer to discern the best descriptor of the evolutionary process. From this, it was best deduced as a vertical genealogy with sporadic lateral gene transfer. Amongst organisms that contain the trp operon are structurally similar enzymes inferring the gen...
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...cture. Regions 3 and 4 are also complementary and can form this same structure. Which of the two structures form is dependent on the level of trp in the environment. If trp is abundant, then as the ribosome translates over region 1, charged tRNA-trp will arrive at the codon site allowing for fast translation and quick arrival and partial overlap of region 2 making it unavailable to associate with region 3. Region 3 then associates with region 4 signaling for termination and for RNA polymerase to disassociate from the DNA before it transcribes the structural genes. When the environment is starved of tryptophan, as the ribosome translates over region 1, the ribosome stalls as it waits for charged tRNA-trp. This delay allows for the association of region 2 with region 3 preventing the pairing of regions 3 and 4 and permitting the transcription of the structural genes.
Miller, Kenneth R. and Joseph S. Levine. “Chapter 12: DNA and RNA.” Biology. Upper Saddle River: Pearson Education, Inc., 2002. Print.
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