This is the shortest sequence for constitutive T7 promoter derived from wild-type T7 bacteriophage. This promoter allows high expression of proteins only when the T7 polymerase is present. Advantage of T7 promoters is that they are not recognized by Escherichia coli RNAP making T7 promoters capable of independent regulation from E. coli promoters. Since an active strong T7 promoter can completely stop the growth of E. coli due to the high level of resource consumption (e.g. nucleotides) by the T7 RNAP, it is typically necessary to regulate the levels of either the T7 RNAP or the T7 promoter or both. The most common cell strain to use with a T7 promoter system is BL21(DE3) which is an E. coli B strain that contains a an inducible T7 RNAP gene on the chromosome.