Because the structural genes in an operon are coordinately expressed, mutations that affect only one enzyme are likely to occur in the structural genes; mutations that affect both enzymes must occur in the promoter or regulator.

4. A mutation occurs in the 5' UTR of the trp operon that reduces the ability of region 2 to pair with region 3. What would be the effect of this mutation when the tryptophan level is high and when the tryptophan level is low?

When the tryptophan level is high, regions 2 and 3 do not normally pair, and therefore the mutation will have no effect. When the tryptophan level is low, however, the ribosome normally stalls at the Trp codons in region 1 and does not cover region 2, and so regions 2 and 3 are free to pair, which prevents regions 3 and 4 from pairing and forming a terminator, ending transcription. If regions 2 and 3 cannot pair, then regions 3 and 4 will pair even when tryptophan is low and attenuation will always occur. Therefore, no more tryptophan will be synthesized even in the absence of tryptophan.

The New Genetics

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