The lac operator as a phenotypic label for DNA fragments cloned in Escherichia coli

To confer a detectable phenotype on any DNA fragment cloned in Escherichia coli, one can label the fragment by ligating it to the lac operator so that host cells can be identified as blue colonies on agar plates. This screening strategy is similar to that used for the pUC and M13 series of vectors, but does not require the vector to contain the lacZ gene. Instead, the presence of the lac operator on the multicopy vector results in the induction of the host cell beta-galactosidase by titrating out the repressor. This paper describes how pUC/M13 vectors or synthetic oligodeoxynucleotides can be used to supply the operator label, and shows how this method has been used to position unique restriction sites for initiating BAL 31 deletions. This approach may be particularly helpful when a given DNA fragment is to be cloned in many different constructs or is to pass through many sequential cloning steps.