Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 144

RNA transcripts are copied only from selected regions of the DNA, rather than from the whole molecule; therefore, the RNA polymerase holoenzyme can start making an RNA chain from a double-stranded DNA only at certain sites. These DNA regions are called promoters, and the RNA polymerase recognizes a particular nucleotide (usually a T or C) in the promoter region of the template strand as a transcription start site, shown as +1 in Figure 2.6. Thus, the first base in the chain is usually an A or a G laid down opposite to a T or C, respectively.

The RNA polymerase holoenzyme recognizes different types of promoters on the basis of which type of σ factor it contains. The most common promoters are those recognized by the RNA polymerase with the σ called σ⁷⁰ in E. coli. The σ factors are often named for their size, and this one has a molecular mass of 70,000 Da (70 kDa). Replacement of σ⁷⁰ with a different σ factor results in an RNA polymerase holoenzyme that recognizes a different set of promoters; this will be discussed in later chapters on gene regulation.

Promoters recognized by holoenzymes containing the same σ are not identical to each other, but they do share certain sequences, known as consensus sequences, by which they can be distinguished. Figure 2.6 shows the consensus sequence of promoters recognized by holoenzymes containing σ⁷⁰ in E. coli, which illustrates a common pattern for promoter structure. The promoter sequence has two important regions: a short AT-rich region centered about 10 bp upstream of the transcription start site, known as the –10 sequence, and a second region centered about 35 bp upstream of the start site, called the –35 sequence. The σ⁷⁰ factor usually must bind to both sequences to start transcription (see below) but does not require that the DNA have a perfect match to these consensus sequences; binding to the promoter occurs only when σ⁷⁰ is in the holoenzyme complex. Sequence-specific binding to the promoter determines not only the site at which transcription will initiate, but also the direction the RNA poiymerase will move along the DNA (in other words, which strand of the DNA will be transcribed from a given region).