A series of new expression vectors (the pTTQ series) has been constructed for the regulated expression of genes in Escherichia coli. Based on the pUC plasmids, the pTTQ vectors contain a polylinker/lacZ alpha region flanked by the strong hybrid trp-lac (tac) promoter and the rrnB transcription terminator. Foreign genes can be inserted into the polylinker region of this expression cassette, to give either transcriptional or translational fusions within the lacZ alpha coding region. In most commonly used strains of E. coli, multiple copies of the lac operator titrate out the lac repressor. This phenomenon leads to significant expression from tac or lac promoters present on multicopy plasmids, even in the absence of inducers such as IPTG. To ensure maximal repression of the tac promoter on the pTTQ vectors in any host strain, the lacIQ allele of the lac repressor gene was added to the vectors. This makes them particularly useful for cloning genes when expression at high level is desired but is detrimental to cell growth.