Mechanism of Actinomycin D inhibition:

• Promoter recognition is mediated by the RNA polymerase sigma factor.
• Actinomycin D contains a phenoxazone ring that intercalates between adjacent G-C base pairs of double-stranded DNA, with its peptide chains sitting in the minor groove; this prevents strand separation and template function, highlighting the importance of proper template strand conformation for transcription.
• Terminator recognition is mediated by rho protein or intrinsic hairpin structures.
• Primer annealing is a replication feature.

Types of eukaryotic RNA polymerases and their transcripts:

• RNA Polymerase I resides in the nucleolus and synthesizes large precursor ribosomal RNA ($28\text{S}$, $18\text{S}$, $5.8\text{S}$ rRNA), matching (III).
• RNA Polymerase II transcribes protein-coding genes to synthesize precursor messenger RNA (mRNA) and most snRNAs, matching (I).
• RNA Polymerase III synthesizes transfer RNA (tRNA), $5\text{S}$ rRNA, and other small nuclear and cytoplasmic RNAs, matching (II).

Catabolite repression signaling in bacteria:

• In bacteria such as E. coli, glucose is the preferred energy metabolite; when glucose is absent or depleted, adenylate cyclase activity increases, producing high levels of cyclic adenosine monophosphate (cAMP) that signals nutrient stress to activate the CAP/CRP activator.
• cGMP is an intracellular second messenger in eukaryotic pathways.
• Inositol phosphates participate in eukaryotic calcium signaling cascades.
• Tyrosine kinases phosphorylate protein tyrosine residues in eukaryotes.

Function of the trp operon:

• The trp operon in E. coli is a repressible operon that encodes five enzymes (TrpE, TrpD, TrpC, TrpB, TrpA) responsible for the anabolic biosynthesis of the essential amino acid tryptophan from chorismate.
• Tryptophan degradation (catabolism) is not mediated by this operon.
• Interconversions are handled by independent enzymes.
• Precursor chorismate synthesis is encoded by the common shikimate pathway genes, not the trp operon.