Two hybrid promoters that are functional in Escherichia coli have been constructed. These hybrid promoters, tacI and tacII, were derived from sequences of the trp and the lac UV5 promoters. In the first hybrid promoter (tacI), the DNA upstream of position -20 with respect to the transcriptional start site was derived from the trp promoter. The DNA downstream of position -20 was derived from the lac UV5 promoter. In the second hybrid promoter (tacII), the DNA upstream of position -11 at the Hpa I site within the Pribnow box was derived from the trp promoter. The DNA downstream of position -11 is a 46-base-pair synthetic DNA fragment that specifies part of the hybrid Pribnow box and the entire lac operator. It also specifies a Shine-Dalgarno sequence flanked by two unique restriction sites (portable Shine-Dalgarno sequence). The tacI and the tacII promoters respectively direct transcription approximately 11 and 7 times more efficiently than the derepressed parental lac UV5 promoter and approximately 3 and 2 times more efficiently than the trp promoter in the absence of the trp repressor. Both hybrid promoters can be repressed by the lac repressor and both can be derepressed with isopropyl beta-D-thiogalactoside. Consequently, these hybrid promoters are useful for the controlled expression of foreign genes at high levels in E. coli. In contrast to the trp and the lac UV5 promoters, the tacI promoter has not only a consensus -35 sequence but also a consensus Pribnow box sequence. This may explain the higher efficiency of this hybrid promoter with respect to either one of the parental promoters.