Tight-binding repressors of the lactose operon

Sixteen mutants which produce lactose repressors with enhanced operator affinities have been isolated. By deletion mapping, six of seven mutations mapped fall into a restricted region of the i gene which also is the location of some anomalous i^s (super-repressor) and some weak i^?d mutations (Pfahl et al., 1974). In vivo and in vitro characterization of nine of the “tight-binding” repressors indicates that: (1) they cause 1.5- to 6-fold decreases in basal β-galactosidase specific activities relative to the parental Q wild-type repressor, and have up to 30-fold increases in operator affinity in vitro. (2) With a few exceptions, the tight-binding repressors show the same relative decreases in basal β-galactosidase specific activities for a wide range of operator types (o⁺ and o^c). (3) With two exceptions, the tight-binding repressors show normal or nearly normal affinities for the inducer, isopropyl-β-d-thiogalactoside, although the concentrations of inducer needed to release various repressors from the o⁺ operator vary greatly.     

Ultraviolet radiation-induced derepression of the lactose operon of E. coli

The effect of ultraviolet irradiation of a regulatory protein, the lac repressor, on its interactions with operator DNA is investigated by spectroscopic and electrophoresis methods. A second set of experiments is performed to assay the capacity of the system containing the irradiated repressor to be induced by IPTG. The protein-nucleic acid interactions are modified upon ultraviolet irradiation of the repressor. The inducer becomes ineffective and repressor stays "locked" to DNA in conditions in which the native repressor is released from the system. These facts are discussed in terms of genes repression and of promotion step in ultraviolet induced carcinogenesis.     

Thiogalactoside transacetylase of the lactose operon as an enzyme for detoxification.

Thigalactoside transacetylase, the lacA gene product, confers selective advantage to cells of Escherichia coli K-12 growing on beta-galactosides in the presence of non-metabolizable analogues.     

Polarity effects in the lactose operon of Escherichia coli

An intergenic RNA segment between lacY and lacA of the lactose operon in Escherichia coli is cleaved by RNase P, an endoribonuclease. The cleavage of the intergenic RNA was ten times less efficient than cleavage of a tRNA precursor in vitro. Fragments of the RNase P cleavage product are detectable in vivo in the wild-type strain but not in a mutant strain at the restrictive temperature. The cleavage product that contains lacA in the wild-type strain was quickly degraded. When this intergenic segment was cloned upstream of a reporter gene, the expression of the reporter gene was also inhibited substantially in wild-type E.coli, but not in a temperature sensitive mutant strain in RNase P at the restrictive temperature. These results support data regarding the natural polarity between lacZ versus lacA, the downstream gene.     

Modulation of the lactose operon mRNA turnover by inhibitors of dihydrofolate reductase.

The decay rate of lactose messenger RNA is strongly decreased when inhibitors of dihydrofolate reductase are added to the growth medium. The effect is not immediate: it appears after one generation of growth in the presence of the inhibitor. Additional genetic evidence suggests that the level of one carbon-tetrahydrofolate might be involved in the processing of mRNA.     

Mutational inversion of control of the lactose operon of Escherichia coli

An unusual Lac regulatory mutant of Escherichia coli K12 has been isolated and studied by genetic, physiological and biochemical techniques. In this mutant galactosides which are inducers for the wild-type organism act as corepressors, shutting off Lac enzyme synthesis. Mapping and dominance tests indicate the mutation is in the i-gene. Both in vivo and in vitro studies demonstrate that Lac repressor is made only in the presence of certain galactosides, and that the product so produced has the normal or wild-type affinity for such galactosides, yet is immune to their normal inducing action. The unusual properties of this mutant make it an excellent tool for determining the relationship between the intracellular repressor concentration and the resulting rate of Lac enzyme synthesis.     

A mutant Ebg enzyme that converts lactose into an inducer of the lac operon.

Lactose is not itself an inducer of the lac operon, nor is it converted to an inducer by ebg+ beta-galactosidase of Escherichia coli. We report here the isolation of a mutant Ebg beta-galactosidase which is capable of converting lactose into an inducer of the lac operon.     

Modeling operon dynamics: the tryptophan and lactose operons as paradigms

Understanding the regulation of gene control networks and their ensuing dynamics will be a critical component in the understanding of the mountain of genomic data being currently collected. This paper reviews recent mathematical modeling work on the tryptophan and lactose operons which are, respectively, the classical paradigms for repressible and inducible operons.