The tac-promoter direct transcription approximately 11 and 7 times more efficiently than the derepressed parental lac 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-Dthiogalactoside (IPTG). 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 promoters, the tac-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