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ISOMERIZATION

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When RNA polymerase holoenzyme first binds to the promoter, the DNA is double stranded. The resulting complex is called the RPc, because the DNA strands are still “closed.” In the next step, the β′ pincer of the crab claw closes around the DNA to form the active-site channel around the template strand of the DNA. This allows the σ2 region to separate the strands of DNA at the –10 region and bind to the nontemplate strand in a process called isomerization (Figures 2.8 and 2.9). Recall that AT base pairs are less stable than GC pairs, so the AT-rich –10 sequence is relatively easy to melt. The complex is now called the open complex (RPo), since the strands of DNA at the –10 region of the promoter are “open.” The +1 nucleotide of the template strand is held in the active-site channel, where the polymerization reaction is about to occur.

Snyder and Champness Molecular Genetics of Bacteria

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