= Alpha-amanitin
<Alpha-amanitin> is a mushroom-derived <cyclic peptide> containing eight <amino acid residues>. It inhibits <RNA polymerase II>, making it a molecular tool for studying <transcription> as well as a naturally occurring toxin. Its small residue count coexists with a constrained ring-and-bridge chemical structure.<Alpha-Amanitin in PubChem>[CID 9543442, Description and structural identity]{ref}
A structural experiment soaked <RNA polymerase II> <transcription elongation complex> crystals with <alpha-amanitin>. The inhibitor trapped a state resolved at $3.4\,\text{Å}$ in which the entering <DNA> base lay above a shifted <RNA polymerase bridge helix> instead of occupying the normal templating position. The authors interpreted this as a possible intermediate in the polymerase's movement along <DNA>.<Structural basis of transcription inhibition by alpha-amanitin and implications for RNA polymerase II translocation by Florian Brueckner and Patrick Cramer>[2008, abstract: crystal soaking, resolution and proposed translocation intermediate]{ref}
This illustrates how blocking a moving <enzyme> can reveal part of its reaction cycle. A trapped structure identifies a physically accessible arrangement; establishing how frequently that arrangement occurs during uninhibited <transcription> requires kinetic evidence as well. Inhibition of <RNA polymerase II> should also be specified by assay conditions rather than treated as a universal claim about every <RNA polymerase>.<Role of DNA-Dependent RNA Polymerase III in the Transcription of the tRNA and 5S RNA Genes by Roberto Weinmann and Robert G. Roeder>[1974, polymerase-dependent alpha-amanitin inhibition comparisons]{ref}
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