(A) The promoters are characterized by two hexameric DNA sequences, the -35 sequence and the -10 sequence named for their approximate location relative to the start point of transcription (designated +1). For convenience, the nucleotide sequence of a single strand of DNA is shown; in reality the RNA polymerase recognizes the promoter as double-stranded DNA. On the basis of a comparison of 300 promoters, the frequencies of the four nucleotides at each position in the -35 and -10 hexamers are given. The consensus sequence, shown below the graph, reflects the most common nucleotide found at each position in the collection of promoters. The sequence of nucleotides between the -35 and -10 hexamers shows no significant similarities among promoters. (B) The distribution of spacing between the -35 and -10 hexamers found in E. coli promoters. The information displayed in these two graphs applies to E. coli promoters that are recognized by RNA polymerase and the major σ factor (designated σ70). As we shall see in the next chapter, bacteria also contain minor σ factors, each of which recognizes a different promoter sequence. Some particularly strong promoters recognized by RNA polymerase and σ70 have an additional sequence, located upstream (to the left, in the figure) of the -35 hexamer, which is recognized by another subunit of RNA polymerase.