The structure of unstable constitutive revertants of mutant galOP-308::IS2-I.

The isolation and characterization of three unstable and constitutive revertants of mutant galOP-308 of E. coli is described. In this mutant an IS2 element is integrated between the promoter and the first structural gene of the galactose operon, and exerts a strong polar effect on the expression of the three galactose genes. In the three revertants under investigation it was observed that relief of polarity and constitutive expression of the gal-operon were accompanied by the deletion of 90% of the IS2 sequence and of various lengths of the adjacent sequences including the gal-promoter. We conclude from this result that the transcription termination signals causing strong polarity were located on the deleted part of IS2, and that in our revertants the galactose genes are now under the control of a new promoter which is apparently unstable.