An artificial 
promoter library (or a set of 
promoter sequences) for a selected 
organism or group of organisms is constructed as a mixture of 
double stranded DNA fragments, the sense strands of which comprise at least two 
consensus sequences of efficient promoters from said 
organism or group of organisms, or parts thereof comprising at least half of each, and surrounding intermediate 
nucleotide sequences (spacers) of 
variable length in which at least 7 nucleotides are selected randomly among the nucleobases A, T, C and G. The sense strands of the 
double stranded DNA fragments may also include a regulatory 
DNA sequence imparting a specific regulatory feature, such as activation by a change in the growth conditions, to the promoters of the 
library. Further, they may have a sequence comprising one or more recognition sites for restriction endonucleases added to one or both of their ends. The selected 
organism or group of organisms may be selected from prokaryotes and from eukaryotes; and in prokaryotes the 
consensus sequences to be retained most often will comprise the −35 
signal (−35 to −30): TTGACA and the −10 
signal (−12 to −7): TATAAT or parts of both comprising at least 3 conserved nucleotides of each, while in eukaryotes said 
consensus sequences should comprise a 
TATA box and at least one upstream activation sequence (UAS). Such artificial 
promoter libraries can be used, e.g., for optimizing the expression of specific genes in various selected organisms.