Stability of cloned promoter-containing fragments.

Strong bacteriophages lambda and T7 promoters for Escherichia coli RNA polymerase were cloned in a multicopy plasmid. To achieve this result, two variants of the promoter-probe vectors were constructed. It was found that (i) modifications of the nucleotide sequence, apart from the commonly accepted promoter region, both upstream and downstream of the RNA initiation point greatly influenced the efficiency of promoters in vivo, (ii) a recombinant DNA composed of one of the promoter-probe plasmids and a tandem of A1, A2, and A3 promoters of T7 bacteriophage DNA induced a reproducible secondary change in plasmid DNA upon cloning. This change was substitution of the part of the recombinant that originated as T7 by a large portion of the host DNA.