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Regulation of gene expression and cell specialization


The trp operon in E. Coli is a coordinately controlled set of genes involved in tryptophan biosynthesis. Tryptophan acts as a corepressor of the operon, which means it binds to and activates the trp repressor protein. This binding is more likely to occur when tryptophan is present at high levels in the cell.
The following diagram represents the genes and regulatory sequences of the trp operon.
Two parallel horizontal lines are shown with seven boxes between the lines. The first box is labeled promoter, the next box is labeled operator, followed by boxes labeled trpE, trpD, trpC, trpB, and trpA. Above the boxes labeled promoter and operator is a bracket with the caption regulatory sequences. Above the boxes labeled trpE, trpD, trpC, trpB and trpA is another bracket with the caption Genes.
The following symbols represent the macromolecules involved in transcriptional regulation of the trp operon.
A key in which a red hexagon attached to a pentagon is labeled tryptophan, a blue oval is labeled trp repressor protein, an orange rectangle is labeled RNA polymerase, and a black squiggly line is labeled mRNA.
Which of the following diagrams best models the regulation of the trp operon when tryptophan is present at high levels?
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