Chapter 16

7. What is attenuation? What is the mechanism by which the attenuator forms when tryptophan levels are high and the antiterminator forms when tryptophan levels are low?

Attenuation is the termination of transcription prior to the structural genes of an operon. It is a result of the formation of a termination hairpin structure or attenuator in the mRNA. Two types of secondary structures can be formed by the mRNA 5′ UTR of the trp operon. If the 5′ UTR forms two hairpin structures from the base pairing of region 1 with region 2 and the pairing of region 3 with region 4, then transcription of the structural genes will not occur. The hairpin structure formed by the pairing of region 3 with region 4 results in a terminator being formed that stops transcription. When region 2 pairs with region 3, the resulting hairpin acts as an antiterminator allowing for transcription to proceed. Region 1 of the 5' UTR also encodes a small protein and has two adjacent tryptophan codons (UGG). Tryptophan levels affect transcription due to the coupling of translation with transcription in bacterial cells. When tryptophan levels are high, the ribosome quickly moves through region 1 and into region 2, thus preventing region 2 from pairing with region 3. Therefore, region 3 is available to form the attenuator hairpin structure with region 4, stopping transcription. When tryptophan levels are low, the ribosome stalls or stutters at the adjacent tryptophan codons in region 1. Region 2 now becomes available to base pair with region 3, forming the antiterminator hairpin. Transcription can now proceed through the structural genes.

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11. For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator. a. Negative control in a repressible operon b. Positive control in a repressible operon c. Negative control in an inducible operon d. Positive control in an inducible operon

a. Inactive repressor b. Active activator c. Active repressor d. Inactive activator

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5. Briefly describe the lac operon and how it controls the metabolism of lactose.

lac operon consists of three structural genes involved in lactose metabolism, the lacZ gene, the lacY gene, and the lacA gene. Each of these three genes has a different role in the metabolism of lactose. The lacZ gene codes for the enzyme β-galactosidase, which breaks the disaccharide lactose into galactose and glucose, and converts lactose into allolactose. The lacY gene, located downstream of the lacZgene, codes for lactose permease. Permease is necessary for the passage of lactose through the E. coli cell membrane. The lacA gene, located downstream of lacY, encodes the enzyme thiogalactoside transacetylase whose function in lactose metabolism has not yet been determined. All of these genes share a common overlapping promoter and operator region. Upstream from the lactose operon is the lacI gene that encodes the lac operon repressor. The repressor binds at the operator region and inhibits transcription of the lac operon by preventing RNA polymerase from successfully initiating transcription. When lactose is present in the cell, the enzyme β-galactosidase converts some of it into allolactose. Allolactose binds to the lac repressor, altering its shape and reducing the repressor's affinity for the operator. Since this allolactose-bound repressor does not occupy the operator, RNA polymerase can initiate transcription of the lac structural genes from the lac promoter.