Pribnow box
The Pribnow box is the sequence TATAAT of six nucleotides (thymine-adenine-thymine-etc.) that is an essential part of a promoter site on DNA for transcription to occur in prokaryotes. It is located roughly 10 base pairs upstream (that is, in the direction 5' from the first transcribed base). It is considered a consensus sequence - that is, it is the most common base sequence to appear at such points on the DNA helix; there may be variations in various organisms. Since adenine and thymine pair together with only two hydrogen bonds (as opposed to three as with guanine and cytosine), they are easier to break apart, making them favourable sites for RNA polymerase to latch onto.
Probability of occurrence of each nucleotide
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