Operon Models Questions and Answers

Answer: C

The original mutation is recessive.

In the merodiploid the cell has one mutant and one wild-type copy of the relevant region. The result is a normal, inducible phenotype, so the wild-type copy supplies what the mutant copy lacks, which is the definition of a recessive mutation. A typical example is a lacI^- mutation: the wild-type lacI on the other DNA makes a diffusible repressor that acts in trans on both operons.

A dominant or trans-dominant mutation (such as lacI^s or lacI^-d) would impose its own phenotype in the diploid. A cis-dominant mutation such as operator-constitutive lacO^c keeps its own operon constitutive in the merodiploid, so it would not give a fully inducible result.

Answer: A

The correct profile is P.

When lactose is added, the repressor is released and transcription starts almost at once, so lac mRNA rises first. Bacterial mRNA is very short-lived (a half-life of a few minutes), so when lactose is removed and transcription stops, the mRNA level falls back quickly.

The enzymes appear slightly later, since they must be translated from the mRNA, and then level off rather than fall: the proteins are stable, so after induction stops they remain, only diluted by growth. β-Galactosidase, from the first gene of the operon, is made in larger amounts than permease, which is encoded further downstream. Profile P shows all three features.

Q puts permease above β-galactosidase. R shows permease being lost within minutes of removing lactose, as if it were as unstable as the mRNA. S has the mRNA rising last and staying high while β-galactosidase disappears, the reverse of the real behaviour of message and protein.

Answer: B

The antiterminator forms between regions 2 and 3 of the trpL leader.

The trp operon is regulated by attenuation. Region 1 of the leader encodes a short peptide with two tandem tryptophan codons. When tryptophan is scarce, charged tRNA^Trp is scarce too, and the ribosome stalls on region 1. Region 2 is then free to pair with region 3, forming the antiterminator, which prevents the 3:4 terminator hairpin from forming, so RNA polymerase continues into the structural genes.

When tryptophan is plentiful, the ribosome moves on and covers region 2, so regions 3 and 4 pair to form the terminator and transcription stops. The 1:2 hairpin is the pause structure. Regions 2 and 4 are not complementary partners in this model.

Answer: B

Expression is activated by the dissociation of bound lac repressor from the operator site and binding of cAMP-CAP complex in the CAP site.

Lactose is converted to allolactose, which binds the lac repressor and releases it from the operator, so RNA polymerase is no longer blocked. When glucose is depleted, cAMP levels rise; cAMP binds CAP, and the cAMP-CAP complex binds the CAP site upstream of the promoter, helping RNA polymerase to bind and strongly activating transcription.

Releasing the repressor alone allows only low-level expression. A repressor still bound blocks transcription, and losing cAMP-CAP from its site is what happens when glucose is present.

Answer: B

The essential genes for lysogeny are CI, CII, CIII.

After lambda infects E. coli, the CII protein activates the promoters for the repressor gene cI and for the integrase gene; CIII protects CII from degradation by the host protease. The CI repressor then shuts down the lytic genes and maintains the prophage, so all three are needed for the lysogenic path.

Cro favours the lytic path, and N is an antiterminator used early in both paths. P and O are replication proteins of the lytic cycle, and xis is needed for excision of the prophage, not for establishing lysogeny.

Answer: B

Such a site is called a Cis acting site.

A cis-acting element, such as a promoter or an operator, is a stretch of DNA that controls only the genes physically adjacent to it on the same molecule. Because it is not a diffusible product, a second copy elsewhere in the cell cannot rescue or override it, so its effect holds irrespective of the other alleles present; this is what cis-trans tests detect.

A trans-acting factor is a diffusible product, usually a protein such as a repressor, that acts on any copy of its target. 'Overlapping site' and 'cis-trans acting site' are not the terms for this property.

Answer: A

The low level is of Cyclic AMP.

When glucose is plentiful, its uptake leaves adenylate cyclase inactive, so cAMP levels fall. Without cAMP, the catabolite activator protein (CAP) cannot bind next to the lac promoter, and RNA polymerase transcribes the operon poorly even if lactose is present. This is catabolite repression.

The LacI repressor responds to lactose (through allolactose), not glucose. CAP is an activator, and it is the shortage of cAMP, not of the protein, that matters. Lactose is not synthesised by the cell.

Answer: D

Expression needs the presence of lactose and absence of glucose.

The structural genes come from the trp operon, but their transcription is now controlled by the lac promoter region. That region responds to two signals: lactose (as allolactose) removes the lac repressor, and the absence of glucose raises cAMP so that CAP-cAMP can activate transcription.

Glucose keeps cAMP low and so blocks efficient expression, and without lactose the repressor stays bound. Tryptophan is irrelevant, because the trp promoter and operator that respond to it are no longer present.

Answer: C

The level of regulation is attenuation.

The trp leader region encodes a short peptide containing two tryptophan codons. When tryptophan is plentiful, the ribosome translates the leader quickly, which lets the transcript fold into a terminator hairpin, and RNA polymerase stops before the structural genes. When tryptophan is scarce, the ribosome stalls and an alternative antiterminator hairpin forms instead.

Corepression is the other control of the trp operon: the repressor, with tryptophan bound, blocks transcription from starting. It acts at initiation, not by terminating transcription. 'Activation' and 'termination' are not the names of this regulatory level.