Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 145

Figure 2.7 shows an overview of the steps of transcription. The RNA polymerase holoenzyme recognizes a promoter and begins transcription with a ribonucleoside triphosphate. As the RNA chain begins to grow, the RNA polymerase holoenzyme releases its σ factor, and the five-subunit core enzyme continues to move along the template DNA strand in the 3′-to-5′ direction, synthesizing RNA in the 5′-to-3′ direction. Inside an opening in the DNA helix approximately 17 bases long, called the transcription bubble, the elongating RNA and the template DNA strand pair with each other to form a DNA-RNA hybrid of approximately 8 or 9 bp with a double-helix structure similar to that of a double-stranded DNA molecule. As RNA polymerase moves along the DNA, the upstream portion of the DNA-RNA helix separates as new ribonucleotides are incorporated into the 3′ end of the growing RNA chain, and the 8-to 9-bp hybrid is maintained. The resulting RNA product emerges from RNA polymerase through a channel, and the DNA strands behind the RNA polymerase rebind to each other. The RNA polymerase continues to move along the DNA template until it reaches a terminator, which signals the RNA polymerase to release both the DNA template and the RNA transcript.

Figure 2.6 (A) Typical structure of a σ⁷⁰ bacterial promoter. (B) The consensus sequences of a σ⁷⁰ bacterial promoter. RNA synthesis typically starts with an A or a G, and no primer is required. N, any nucleotide. (C) Positions of interaction between RNA polymerase and promoter DNA.