Consensus sequence

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Diagram of E.Coli promoter including consensus sequence (figure obtained from Oxford Genetics)
A consensus sequence is an ideal promoter sequence in DNA - in E. coli, for example, two are found, a -35 sequence and a -10 sequence. The ideal promoter sequence - the consensus sequence - is never actually found in DNA, and a promoter's strength can be judged by it's similarity to the consensus sequence. The closer a promoter is to the ideal sequence, the stronger it will be and therefore the more mRNA will be produced, which will lead to a greater yield of proteins. The -35 consensus sequence is TTGACA, and the -10 consensus sequence is TATAAT.

The consensus sequence was originally determined by comparison of promoter sequences that were already known, and selection of the base that was most common at each position. Any sequence upstream of the transcription start site is given a negative sign in front of it as the start site is effectively +1.

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