Summary The
gal3 mutation of
E. coli 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 revertants, 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
+ revertants obtained from a
gal3() strain. Although the constitutive reversions could be transduced 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 .In order to facilitate the packaging of these reversions, the
chlD-pgl region was deleted from the parent
gal3() 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
+(), or
gal3 strains. Moreover, the frequency of spontaneous
chlD-pgl deletions increased 10- to 15-fold in presence of the
gal3 insertion.A
gal phage bearing a true stable, constitutive reversion (
gal
c
200) was isolated from the revertant strain by subsequent deletion of the
chlD-pgl segment (31). Electron micrographs of
gal
+ and
gal
c
200 31(chlD pgl) DNA heteroduplexes were interpreted to indicate that the stable, constitutive reversion had arisen by a deletion of 3/4 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 endonuclease. It is suggested that inhibition of transducing particle formation by
gal3 and the preferred termination of deletions at
gal3 might represent related phenomena.
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