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At least 163 records · Page 9Linked to original sources
PubMed1969

DNA-directed peptide synthesis. VI. Regulating the expression of the lac operon in a cell-free system.

The operon model was proposed by Jacob and Monod in 1961 to explain the regulation of enzyme synthesis in bacteria. The model requires two genetic elements: a locus i which directs the synthesis of a repressor, and a locus called the operon which contains an operator gene and associated structural genes. The repressor is believed to stop RNA and enzyme synthesis by combining with the operator site. The inhibiting effect of the repressor is reversed by an inducer which interacts directly with the repressor. A DNA-dependent cell-free system has been developed for studying the regulation of the operon containing the gene for beta-galactosidase enzyme. In this cell-free system, gene activity is indirectly measured by the synthesis of part of the enzyme. Qualitatively, the regulation of the operon in the cell-free system is similar to its regulation in intact cells. The cell-free system is used to advantage in experiments where it is desirable to use predetermined levels of repressor, operon, and inducer. Such quantitative studies suggest that two inducer molecules are most effective in removing the repressor from the gene.

Cell-Free System↗
PubMed1970

Cyclic 3',5'-adenosine monophosphate and N-acetylglucosamine-6-phosphate as regulatory signals in catabolite repression of the lac operon in Escherichia coli.

When an Escherichia coli mutant lacking the enzyme N-acetyl-glucosamine-6-phosphate (AcGN6P) deacetylase is grown in a succinate-mineral salts medium and exposed to an exogenous source of N-acetylglucosamine, approximately 20 to 30 pmoles of AcGN6P per mug of cell dry weight will accumulate in these cells. This accumulation occurs within 2 to 4 min after the addition of N-acetylglucosamine and is coincident with the production of a severe permanent catabolite repression of beta-galactosidase synthesis. This repression does not occur if adenosine 3',5'-cyclic phosphate (cyclic AMP) is added to the cells before AcGN6P accumulates. An immediate derepression occurs when cyclic AMP is added to cells that have already accumulated a large AcGN6P pool. These findings are consistent with the view that low-molecular-weight carbohydrate metabolites and cyclic AMP play key roles in the catabolite repression phenomenon, and that metabolites such as AcGN6P may participate in the represion mechanism by influencing either the formation or degradation of cyclic AMP in E. coli.

Amidohydrolases↗
PubMed2006

Multiple pathways of selected gene amplification during adaptive mutation.

In a phenomenon referred to as "adaptive mutation," a population of bacterial cells with a mutation in the lac operon (lac-) accumulates Lac+ revertants during prolonged exposure to selective growth conditions (lactose). Evidence was provided that selective conditions do not increase the mutation rate but instead favor the growth of rare cells with a duplication of the leaky lac allele. A further increase in copy number (amplification) improves growth and increases the likelihood of a sequence change by adding more mutational targets to the clone (cells and lac copies per cell). These duplications and amplifications are described here. Before selection, cells with large (134-kb) lac duplications and long junction sequences (>1 kb) were common (0.2%). The same large repeats were found after selection in cells with a low-copy-number lac amplification. Surprisingly, smaller repeats (average, 34 kb) were found in high-copy-number amplifications. The small-repeat duplications form when deletions modify a preexisting large-repeat duplication. The shorter repeat size allowed higher lac amplification and better growth on lactose. Thus, selection favors a succession of gene-amplification types that make sequence changes more probable by adding targets. These findings are relevant to genetic adaptation in any biological systems in which fitness can be increased by adding gene copies (e.g., cancer and bacterial drug resistance).

Adaptation, Biological↗
PubMed1999

Hinge-helix formation and DNA bending in various lac repressor-operator complexes.

The hinge-region of the lac repressor plays an important role in the models for induction and DNA looping in the lac operon. When lac repressor is bound to a tight-binding symmetric operator, this region forms an alpha-helix that induces bending of the operator. The presence of the hinge-helices is questioned by previous data that suggest that the repressor does not bend the wild-type operator. We show that in the wild-type complex the hinge-helices are formed and the DNA is bent, similar to the symmetric complex. Furthermore, our data show differences in the binding of the DNA binding domains to the half-sites of the wild-type operator and reveal the role of the central base-pair of the wild-type operator in the repressor-operator interaction. The differences in binding to the operator half-sites are incorporated into a model that explains the relative affinities of the repressor for various lac operator sequences that contain left and right half-sites with different spacer lengths.

Bacterial Proteins↗