A series of synthetic promoters, based upon the Escherichia coli σ⁷⁰ consensus promoter sequence, was constructed upstream of the lacZ reporter gene in the modified broad-host-range vector pQF52. The role of the intervening spacer region in gene expression in Pseudomonas aeruginosa and E. coli was studied by insertions and deletions within this region. In P. aeruginosa and E. coli the patterns of gene expression were identical with maximum β-galactosidase activity being measured from promoters possessing 19 bp in their intervening regions, presumably as a result of impeded promoter clearance with the consensus 17-bp promoter. In P. aeruginosa a second occurrence of enhanced activity, which could not be attributed to the involvement of the alternative sigma factor RpoN (σ⁵⁴), was evident with the promoter having a 16-bp spacer.Key words: Pseudomonas aeruginosa, promoter, RpoD, RpoN, transcription.