Actinomycin inhibits DNA transcription.
The Phenoxazone ring is a flat structure and sticks between the two chains of the DNA helix at GC base pairs (called intercalation) and the cyclic peptides (the other names in the figure) bind to the narrow groove of the helix. Because it is squeezed into the helix, it isn't possible to transcribe the DNA into mRNA.

At low concentrations, actinomycin D appears to inhibit transcription in prokaryotes as wel as eukaryotes, without affecting DNA replication or protein synthesis itself. The specific activity of it is not only used as an antibiotic, but also during treatment of some cancers (becaue of this inhibition of growth of rapid dividing cells).
Actinomycin originates from the Streptomyces family.

Structure of Actinomycin D
	  O               O
	  ||              ||
  ------- C                C-----------------
  |       |                |		|
  |  L-Methylvaline    L-Methylvaline	|
  |       |		|		|
  |     O=C		C=O		|
  |       |		|		|
  |   Sacrosine	       Sacrosine            |
  |       |		|		|
  |     O=C		C=O		|
  |       |		|		|
  |   L-Proline	        L-Proline           |
  |       |		|		|
  |     O=C		C=O		|
  |       |		|		|
  |   D-Valine               D-Valine         |
  |       |		|		|
  |     O=C                C=O		|
  |       |		|		|
  |---L-Treonine            L-Treonine--------|
          |		|
          C=O             O=C
          |		|
	 Phenoxazone ring


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