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In bacterial promoters subject to stringent control the heptanucleotide sequence spanning nucleotide positions -5 to +2 (defining the -10 region D. Pribnow, J. Mol. Biol. 99:419-443, 1975 as positions -12 to -6) is highly conserved. This conserved region, for which the best consensus sequence is Formula: see text is absent from promoters which are not under stringent control.
Andrew Travers (Fri,) studied this question.