ATP and ATPgS: Two "Competing" Reagents

ATP is essential for the transcription reaction not only because it is a building block for RNA synthesis, but also as an energy source for the ATP-dependent DNA-helicase activities of TFIIH during the Initiation and Promoter Escape stages of transcription. A useful tool to investigate the ATP-requiring steps is ATPgS, which is an analog of ATP with a sulfur atom replacing one of the oxygen atoms in the g phosphate area. This reagents competes with ATP for binding to the active site of the helicase, but cannot be hydrolyzed like ATP, leaving the helicase unable to extract the energy necessary for its activity. Addition of ATPgS to a transcription reaction is equivalent to removal of ATP.

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