Patent ID: 6964845

Claim:
A method for selecting an OR or OL operator DNA sequences from a lambdoid phages wherein said sequence has a different thermostability compared to a wild-type sequence with regard to binding a repressor, wherein said different thermostability results in repression of expression of a gene which is operatively linked to said OR or OL operator DNA sequence from a lambdoid phage until a temperature is reached that is 3 to 10° C. higher than the temperature at which the wild type sequence is capable of repressing the expression of a gene operatively linked thereto, comprising (a) preparing a DNA cassette which contains a selection gene under the operative control of an expression control sequence comprising at least one OR or OL operator DNA sequence from a lambdoid phage and a promoter, (b) intentionally subjecting the operator DNA sequence to a mutagenesis, and (c) analyzing the operator DNA sequences to determine whether said sequences have a different thermostability as compared to a wild-type sequence with regard to binding a repressor, wherein a mutated operator sequence is selected that represses expression of the gene operatively linked thereto at a temperature that is 3 to 10° C higher than the temperature at which the wild type sequence is capable of repressing the expression of a gene operatively linked thereto.