Lactose repressor ( English Lac repressor ) - , which inhibits the expression of genes encoding proteins of the lactose operon . Coded by the lacI gene. Proteins of the lactose operon are involved in the metabolism of lactose in bacterial cells . These genes are suppressed when lactose is inaccessible to cells, ensuring that the bacterial cell will not spend energy on the synthesis of proteins that metabolize lactose, in the absence of it. When lactose becomes available, it is converted to , which inhibits the ability of the lactose repressor to bind to DNA. When the repressor is not associated with the operator of the lactose operon, its transcription and subsequent synthesis of enzymes of the lactose metabolism begin [1] .
| Lactose repressor | |
|---|---|
| Identifiers | |
| Symbol | Laci |
| Pfam | PF00356 |
| Available protein structures | |
| Pfam | the structure |
| PDB | RCSB PDB ; PDBe ; PDBj |
| PDBsum | 3D model |
Content
Structure
Lactose repressor structure
The lactose repressor ( lac- repressor) of Escherichia coli Escherichia coli is a homotetramer weighing 154,520 daltons . Each of the four monomers contains 360 amino acid residues and consists of an N-terminal domain , a hinge or linker site, a sugar binding domain, and a C-terminal domain. The N-terminal domain contains the spiral-rotate-spiral , which is responsible for interaction with the operator. This motif is formed by two α-helices (amino acid residues 6–25). The N-terminal domain is a small, compact globular domain with a pronounced , which creates three α-helices. The linker, or hinge, site (residues 46–62) connect the DNA- binding N-terminal domain to the sugar-binding part (core) of the repressor. It is believed that this site does not have a pronounced secondary structure and consists of disparate helices, however, in the presence of DNA, it is ordered and forms an α-helix that interacts with the operator and in a certain way orientates the DNA-binding domain of the lac- repressor. The repressor core, or sugar-binding domain, consists of two subdomains. The spatial structure of subdomains is very similar, although there is little similarity in the composition of amino acid residues. Each subdomain contains 6 parallel β-sheets sandwiched between four α-helices. The C-terminal domain is responsible for the assembly of the tetramer [1] .
Lactose repressor is an unusual tetramer. Its monomers form stable dimers . This interaction is provided by five clusters of amino acids. Dimers, in turn, can combine to form tetramers due to the interaction of C-terminal alpha-helices (residues 340–357). Each spiral contains two sections, consisting of seven leucine residues , which provide the interaction of the four alpha helices. Lactose repressor tetramers are more correctly considered as dimers of dimers, since they do not have the symmetry characteristic of other oligomeric proteins. It is dimers that bind to DNA, that is, each tetramer of the lactose repressor can be associated with two operators [1] .
Operation
As mentioned above, the binding of the lactose repressor to DNA occurs through the N-terminal helical-rotational-helical structural motif, which binds to the . In addition, hinge regions bind to DNA. The binding occurs due to the interaction of ordered helices of the hinge regions and the small groove of DNA [2] . Since each tetramer can bind two operators simultaneously, the binding of several sequences of operators with one tetramer causes DNA stranding [3] . The binding of the repressor to DNA increases the affinity of the RNA polymerase to the promoter so that it cannot leave it; therefore, the transcription of the lactose operon genes cannot begin to elongate. In the presence of lactose, allolactose binds to the lac- repressor, altering its spatial structure so that the repressor is incapable of strong binding to the operator. In studies conducted in vitro , isopropyl-β-D-1-thiogalactopyranoside (IPTG) is often used as a substance imitating the effect of allolactose [1] .
Opening
The lactose repressor was first Walter Gilbert and Benno Müller-Hill in 1966. Walter Gilbert and Benno Müller-Hill This happened a year after Jacques Monot and Francois Jacob describing the lactose operon received the Nobel Prize in Physiology or Medicine for their studies in the regulation of gene expression. Gilbert and Muller Hill were able to demonstrate in vitro that the protein binds to DNA containing the lactose operon and is separated from the DNA when IPTG is added. They also isolated a portion of the DNA bound to the protein using a deoxyribonuclease enzyme that breaks down DNA. After treatment of the repressor-DNA complex with this enzyme, some DNA molecules remained unsplit; it was suggested that they were protected by the repressor from the action of the enzyme, which was later confirmed. These experiments confirmed the mechanism of the lactose operon, previously proposed by Mono and Jacob [4] [1] .
Notes
- ↑ 1 2 3 4 5 Lewis M. The lac repressor. (English) // Comptes rendus biologies. - 2005. - Vol. 328, no. 6 . - P. 521-548. - DOI : 10.1016 / j.crvi.2005.04.004 . - PMID 15950160 .
- ↑ Schumacher MA , Choi KY , Zalkin H. , Brennan RG Crystal structure of LacI member, PurR, bound to DNA: minor groove binding by alpha helices. (English) // Science (New York, NY). - 1994. - Vol. 266, no. 5186 . - P. 763-770. - PMID 7973627 .
- ↑ Oehler S. , Eismann ER , Krämer H. , Müller-Hill B. The three operators of the lac operon cooperate in repression. (Eng.) // The EMBO journal. - 1990. - Vol. 9, no. 4 . - P. 973-979. - PMID 2182324 .
- ↑ Gilbert W. , Müller-Hill B. Isolation of the lac repressor. (Eng.) // Proceedings of the National Academy of Sciences of the United States of America. - 1966. - Vol. 56, no. 6 . - P. 1891-1898. - PMID 16591435 .