Melee. gen. Genet. 142, 263--275 (1975) © by Springer-Verlag 1975 Reversion of the gal3 Mutation of Escherichia coli: Partial Deletion of the Insertion Sequence Asad Ahmed and Eric Johansen Department of Genetics, University of Alberta, Edmonton, Alberta T6G 2E9, Canada Received October 6, 1975 Summary. The gal3 mutation of E. cell is an insertion of a DNA sequence, 1,100 base pairs in length, into the operator-promoter region of the galactose operon. This mutation reverts spontaneously to gal+ by excision of the insertion to produce stable, inducible re- vertants, or by tandem duplications of the gal operon to produce unstable, constitutive revertants. The nature of a third class of revertants, which are stable and constitutive, is the subject of the present study. The stable, constitutive class of revertants included approximately 30% of all gal+ re- vertants obtained from a gal3(2) strain. Although the constitutive reversions could be trans- duced by ~, the efficiency was found to be extremely poor and the rare transductants which did appear seemed to originate from abnormal transducing particles. It was concluded that these reversions were not normally packaged by 2. In order to facilitate the packaging of these reversions, the chlD-pgl region was deleted from the parent gal3(Z) strain. Unexpectedly, the gal3 mutation in the majority of these deletions reverted to produce stable, constitutive reversions exclusively. The explanation proposed was that the chlD-pgl deletions had also removed part of the gal operator-promoter up to the gal3 insertion, so that simple excisions of the insertion yielded stable, constitutive revertants by connecting the gal structural genes to a different promoter. These revertants were not considered to be true representatives of the stable, constitutive class. The specificity of deletion end-points at the insertion was found only in the gal3(~) strain, and not in gal+, gal+(X), or gal3 strains. Moreover, the frequency of spontaneous chlD-pgl deletions increased 10- to 15-fold in presence of the gal3 insertion. A 2gal phage bearing a true stable, constitutive reversion (galc200) was isolated from the revertant strain by subsequent deletion of the chID-pgl segment (LI31). Electron micrographs of ~gal+ and 2gale200A31(chID pgl) DNA heteroduplexes were interpreted to indicate that the stable, constitutive reversion had arisen by a deletion of 8/4 of the gal3 insertion sequence. The main conclusions are: (i) the stable, constitutive reversions of gal3 can arise by partial deletions of the insertion sequence, apparently by elimination of the nucleotide sequence which causes polarity; (ii) the chlD-pgl deletions may exhibit preferential termination at the right extremity of the gal3 insertion in presence of prophage ~; and (iii) the gal3 insertion appears to inhibit the production of ~gal particles by providing a nucleotide sequence which is recognized and degraded by a specific endonucleasc. It is suggested that inhibition of trans- ducing particle formation by gal3 and the preferred termination of deletions at gal3 might represent related phenomena. Introduction We have recently shown that the gal3 mutation of Escherichia cell, first described by Morse, Lederberg, and Lederberg (1956), was caused by the in- sertion of a DNA sequence, 1,100 nucleotide pairs in length, in the operator- promoter (OP) region of the galactose (gal) operon (Ahmed and Scraba, 1975). This finding provided satisfac¢ory explanation for the various unusual features