Alterations in two conserved regions of promoter sequence lead to altered rates of polymerase binding and levels of gene expression.

Characterization of recombinant lac promoters highlights the importance of two regions of sequence conservation in promoters. The "Pribnow box" sequences are necessary for specific transcription in this system. This specificity is maintained when a mutated upstream sequence is introduced. However, changing the upstream DNA sequences influences both the rate of RNA polymerase binding in vitro and levels of expression in vivo.