The region preceding the beta-lactamase promoter of Escherichia coli plasmid pUC19 has a DNA curvature (curved DNA or bent DNA). Studies using mutant plasmids showed that the curved DNA structure is required for efficient transcription of the beta-lactamase gene and suggested that the requisite shape is a small part of a right-handed coil. Based on these results, the possible functional significance of DNA curvature in transcription will be discussed.