| lactose repressor | Protein (tetramer of 37 kD subunits) that normally binds with very high affinity to the operator region of the lactose operon and inhibits transcription of the downstream genes by blocking access of the polymerase to the promoter region. When the lactose repressor binds allolactose, its binding to the operator is reduced and the gene set is derepressed. (18 Nov 1997) |
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| active repressor | A repressor that combines directly with an operator gene to repress the operator and its structural genes, thus repressing protein synthesis; active repressor may be repressed by an inducer, with resulting protein synthesis; a homeostatic mechanism for regulation of inducible enzyme systems. (05 Mar 2000) |
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| repressor | A type of protein molecule that binds to DNA that shuts up transcription of a gene. See: operon or operator. (09 Oct 1997) |
| repressor gene | A gene that prevents a nonallele from being transcribed. (05 Mar 2000) |
| repressor protein | <molecular biology> A protein that binds to an operator of a gene preventing the transcription of the gene. The binding affinity of repressors for the operator may be affected by other molecules. Inducers bind to repressors and decrease their binding to the operator, while co repressors increase the binding. The paradigm of repressor proteins is the lactose repressor protein that acts on the lac operon and for which the inducers are _ galactosides such as lactose, it is a polypeptide of 360 amino acids that is active as a tetramer. Other examples are the lambda repressor protein of lambda bacteriophage that prevents the transcription of the genes required for the lytic cycle leading to lysogeny and the cro protein, also of lambda, which represses the transcription of the lambda repressor protein establishing the lytic cycle. Both of these are active as dimers and have a common structural feature the helix turn helix motif that is thought to bind to DNA with the helices fitting into adjacent major grooves. (18 Nov 1997) |
| repressor proteins | Proteins which are normally bound to the operator locus of an operon, thereby preventing transcription of the structural genes. In enzyme induction, the substrate of the inducible enzyme binds to the repressor protein, causing its release from the operator and freeing the structural genes for transcription. In enzyme repression, the end product of the enzyme sequence binds to the free repressor protein, the resulting complex then binds to the operator and prevents transcription of the structural genes. (12 Dec 1998) |
| met repressor operator complex | Repressor protein, 104 residues, product of the metJ gene, which regulates methionine biosynthesis in E. Coli. Dimeric molecules bind to adjacent sites 8 base pairs apart on the DNA, sequence recognition is by interaction between antiparallel strands of protein and the major groove of the B form DNA duplex. (18 Nov 1997) |
| inactive repressor | A repressor that cannot combine with an operator gene until it has combined with a corepressor (usually a product of a protein pathway); after activation, the repressor arrests production of the proteins controlled by the operator gene; a homeostatic mechanism for regulation of repressible enzyme systems. Synonym: aporepressor. (05 Mar 2000) |
| translational repressor | A molecule that blocks translation by binding to a messenger RNA. (09 Oct 1997) |
| oral lactose tolerance test | A test for lactose deficiency; the plasma glucose response to an oral lactose load is measured as in the (oral) glucose tolerance test. (05 Mar 2000) |
| lactose | The major sugar in human and bovine milk. Conversion of lactose to lactic acid by Lactobacilli etc. Is important in the production of yoghurt and cheese. (18 Nov 1997) |
| lactose carrier protein | <protein> The best known example is the product of the lacY gene, coded for in the lactose operon and responsible for the uptake of lactose by E. Coli. (18 Nov 1997) |
| lactose factors | Plasmids which determine the ability of a bacterium to ferment lactose. (12 Dec 1998) |
| lactose intolerance | A disorder characterised by abdominal cramps and diarrhoea after the consumption of food containing lactose (for example milk, ice cream), believed to occur due to a deficiency of intestinal lactase (enzyme that breaks down lactose), may appear first in young adults who have previously tolerated milk well as infants. (27 Sep 1997) |
| lactose-litmus agar | Agar made by adding 2% lactose and litmus to acid-free nutrient agar; formerly used in the identification of Salmonella typhi. (05 Mar 2000) |
| lactose operon | Group of adjacent and coordinately controlled genes concerned with the metabolism of lactose in E. Coli. The lac operon was the first example of a group of genes under the control of an operator region to which a lactose repressor binds. When the bacteria are transferred to lactose containing medium, allolactose (which forms by transglycosylation when lactose is present in the cell) binds to the repressor, inhibits the binding of the repressor to the operator and allows transcription of mRNA for enzymes involved in galactose metabolism and transport across the membrane (_ galactosidase, galactoside permease and thiogalactoside transacetylase). (18 Nov 1997) |