Mechanism of reversion of the gal3 mutation of Escherichia coli

The gal3 mutation is an insertion of a DNA sequence in the operator-promoter region of the galactose operon of E. coli. It reverts spontaneously to produce three kinds of gal+ revertants, which are: (i) stable and inducible, (ii) stable and constitutive, and (iii) unstable and constitutive. The constitutive revertants also show drastically reduced frequencies of transduction with lambda. The mechanism by which these reversions occur has remained unknown. It is proposed that the stable and inducible revertants arise by accurate excision of the insertion sequence. The unstable and constitutive revertants arise by tandem duplications of the gal operon in such a way that the structural genes of the extra copy of gal operon become connected to a different promoter. The resulting tandem configuration (gal3 ETK...P'E'T'K') permits constitutive expression and gal3 segregation (by internal recombination) simultaneously. The proposal was tested by comparison of the buoyant densities in CsCl of derivatives of a lambdagal phage carrying gal+, gal3, and the inducible and constitutive revertants. The densities of the inducible revertants were identical to the wild type, and the slight increase in density found to be associated with the gal3 insertion was missing. It was concluded that inducible revertants arise by excision of the inserted sequence. In contrast, lysates of a constitutive revertant exhibited several anomalous properties. The lysates contained a small quantity of phage whose density was identical to lambdagal3, produced few gal+ transductants (10(-3)-10(-4) of a normal HFT lysate), and the transductants were stable and constitutive. In turn, these abnormal transductants produced lysates which showed no lambdagal particles on centrifugation, and no transducing activity whatsoever. These anomalous properties of the constitutive revertant were attributed to the failure of lambda to package the DNA duplication efficiently. Transduction experiments with P1 (which can package more DNA than lambda) show that the unstable, constitutive reversions were located adjacent to prophage lambda. Segregation of the gal and lambda markers among the gal+ transductants was in accordance with the pattern expected for a duplication. Introduction of a recA marker resulted in stabilization of the reversion without affecting its constitutive expression. It was concluded that the unstable, constitutive reversion was a tandem duplication. It is further proposed that the stable, constitutive class of revertants might represent inverted (gal3 ETK...K'T'E'P') or partial tandem (gal3 ET...E'T'K') duplications of the gal operon.     

A mutation defining ultrainduction of the Escherichia coli gal operon.

Tn10 insertion in the galS (ultrainduction factor) gene of Escherichia coli allows the gal operon to be constitutively expressed at a very high level, equal to that seen in a delta galR strain in the presence of an inducer. The insertion has been mapped by criss-cross Hfr matings and by marker rescue into Kohara phages at 46 min on the E. coli chromosome.     

Non- pleiotropic nature of the gal 3 mutation in yeast.

The pleiotropic nature of three independant isolates of yeast galactose long-term adaptation mutants (termed $\text{gal}$ 3) were investigated with respect to the effect of the mutation on the adaptation time of the mutants to their utilization of maltose. Enzyme induction and growth studies with homozygous diploids of the isolates and heterozygous diploid of two of the isolates demonstrate that the mutants readily induce for maltase and grow without delay on maltose as the sole source of carbon and energy, yet still exhibit the long-term adaptation phenotype with respect to the utilization of galactose.     

Galactose metabolism in gal mutants of Salmonella typhimurium and Escherichia coli

Abstract We report a new pathway for galactose metabolism in Escherichia coli and Salmonella typhimurium . Growth of gal mutants on galactose is restored by the addition of pyrrolo-quinoline quinone (PQQ) to the medium. In such strains galactose is oxidized to galactonate by a PQQ-dependent, membrane-bound dehydrogenase. A pathway for galactonate metabolism in these organisms has already been described.     

Disco-ordinate expression of the Escherichia coli gal operon after prophage lambda induction.

EcoRI endonuclease crystallizes in space group C2 with unit cell parameters $\text{a = 209}\text{A}\text{?}\text{, b = 129}\text{A}\text{?}\text{, c = 50}\text{A}\text{?}\text{and}\text{β = 98.4 °}$. Four 29,000 molecular weight subunits per asymmetric unit would give a reasonable V_m value of 2.87 ų/dalton. EcoRI endonuclease is the first protein which recognizes a specific sequence of bases in DNA to be crystallized in a form suitable for high resolution structure analysis.