Prokaryotic gene control signals can be isolated, compared, and characterized by precise fusion in vitro to the Escherichia coli galactokinase gene (galK), which provides both a simple assay and genetic selection. This recombinant galK fusion vector system was applied to the study of promoters and terminators recognized by the Escherichia coli RNA polymerase. Three promoters created by mutation from DNA sequences having no promoter function were characterized. Mutations that inactivate promoter function were selected, structurally defined, and functionally analyzed. Similarly, transcription termination was examined, and mutations affecting terminator function were isolated and characterized.