We have selected a group of Escherichia coli promoters from random DNA sequences by replacing 19 base pairs at the -35 promoter region of the tetracycline resistance gene tetr of the plasmid pBR322. Substitution of 19 base pairs with chemically synthesized random sequences results in a maximum of 4(19) (about 3 X 10(11)) possible replacement sequences. From a population of about 1000 bacteria harboring plasmids with these random substitutions, tetracycline selection has revealed several functional -35 promoter sequences. These promoters have retained only partial homology to the -35 promoter consensus sequence. In three of these promoters, the consensus alignment shifts 10 nucleotides downstream, allowing the RNA polymerase to recognize another Pribnow box from within the original pBR322 sequence. Two of the sequences promote transcription more strongly than the native promoter. This technique may have application for the selection of additional DNA sequences with varied biological activity.