Regulation of Gene Expression Questions and Answers

Answer: B

Catabolite repression is regulated by the concentration of glucose.

When glucose is plentiful, bacteria such as E. coli use it first and repress the genes for using other sugars. In E. coli, glucose transport through the phosphotransferase system lowers adenylate cyclase activity, so cAMP falls; without cAMP the CRP (CAP) activator cannot bind its sites, and operons such as lac are transcribed poorly. Glucose uptake also causes inducer exclusion.

Lactose is the inducer of the lac operon, not the regulator of catabolite repression. Amino acid levels regulate attenuation and the stringent response, and mRNA concentration is an outcome of regulation, not its signal.

Answer: B

Enhancers can act at variable distances from the promoters and may function upstream, downstream, or within introns of genes.

Enhancers are DNA elements bound by activator proteins. They can sit many kilobases from the gene they control, on either side of it or inside an intron, and they work in either orientation. DNA looping brings the bound activators close to the promoter, where they recruit coactivators such as Mediator, which in turn help RNA polymerase II assemble there.

So enhancers do not need to lie immediately upstream or in a fixed orientation. They act through bound proteins rather than by binding RNA polymerase II themselves, and they are separate from the core promoter where the polymerase binds.

Answer: C

The best explanation is that histone acetylation neutralizes positive charge on histones, thereby loosening DNA-histone interactions and promoting transcription.

Histone tails are rich in lysine, whose positively charged side chains hold the negatively charged DNA tightly. Histone acetyl transferases add acetyl groups to these lysines, removing their charge, so the DNA is held less tightly and the chromatin opens for transcription factors and RNA polymerase. Blocking the enzyme leaves the lysines charged, so the chromatin compacts and transcription of many genes falls, exactly what the drug does.

Acetylation removes positive charge rather than adding it. H1 is the linker histone outside the nucleosome core, and its acetylation does not directly dismantle nucleosomes. Euchromatin is the decondensed, active form, so the last option has it backwards.

Answer: B

Nitrogen assimilation genes are transcribed with σ 54.

In E. coli, σ⁵⁴ (RpoN, also called σ^N) directs RNA polymerase to promoters of nitrogen-regulated genes such as glnA, which encodes glutamine synthetase. Unusually, the σ⁵⁴ holoenzyme cannot open the promoter on its own; it needs an ATP-hydrolysing enhancer-binding activator, which for nitrogen genes is NtrC, switched on when nitrogen is scarce.

σ⁷⁰ is the housekeeping factor for most genes in growing cells. σ²⁸ (FliA) transcribes flagellar and chemotaxis genes. σ²⁴ (σ^E) responds to stress in the cell envelope.