Lac operator

Boelens, R., et al. Complex of lac repressor headpiece with a 14 base-pair lac operator fragment studied by two-dimensional nuclear magnetic resonance. /. Mol. Biol. 193 213-216, 1987. 

Cronin CA, Gluba W, Scrable H (2001) The lac operator-repressor system is functional in the mouse. Genes Dev 15 1506-1517... 

F. Culard, M. Schnarr, and J. C. Maurizot, Interaction between the lac operator and the lac repressor headpiece Fluorescence and circular dichroism studies, EMBO J. 1, 1405-1409 (1982). 

In bacteria, genes that encode products with interdependent functions are often clustered in an operon, a single transcriptional unit. Transcription of the genes is generally blocked by binding of a specific repressor protein at a DNA site called an operator. Dissociation of the repressor from the operator is mediated by a specific small molecule, an inducer. These principles were first elucidated in studies of the lactose (lac) operon. The Lac repressor dissociates from the lac operator when the repressor binds to its inducer, allolactose. 

Negative Regulation Describe the probable effects on gene expression in the lac operon of a mutation in (a) the lac operator that deletes most of 0 (b) the lad gene that inactivates the repressor and (c) the promoter that alters the region around position -10. 

The broad peaks B, D, and E are shifted far upfield by reaction with bisulfite (Eq. 5-11) suggesting that they are not hydrogen bonded and are present in the loop of the stem-loop structure. Peaks A, E, F, and G correspond to resonances 64, 7, 67, and 4, respectively, in (A) and represent fluorouracil in the stem structure. From Chu et al.69i Courtesy of Jack Horowitz. (C) A 31P NMR spectrum of a synthetic 14 base-pair DNA segment related to the E. coli lac operator. The palindromic sequence is TCTGAGCGCTCAGA. The numbers refer to the positions from the 5 end. From Schroeder et al.688... 

In addition to the main lac operator Ou which is marked in Fig. 28-2, there are two weaker auxiliary operator sequences designated 02 and 03 located 401... 

The organism senses whether glucose is available by another regulatory mechanism which cooperates with the lac repressor and the lac operator. The promoter is therefore sub-divided into two specific regions, each of distinctive function. One is the RNA polymerase entry site, where RNA polymerase first becomes bound to DNA (cf. DNA transcription, Appendix 5.6), and the other is the protein binding site for the catabolite activator protein (CAP) (Fig. 5.39). The CAP protein binding site controls the polymerase site which, when bound to the DNA, allows successful transcription provided that the repressor is not bound. When the CAP protein is not bound, then RNA polymerase cannot bind and transcription cannot take place. 

The lac repressor has an on rate constant for the binding of the lac operator (when cloned into A) of about 5 x 101om s i. This value is much greater... 

Ogata R, Gilbert W (1977) Contacts between the lac repressor and thymines in the lac operator. Proc Nat Acad Sci USA 74 4973-4976... 

Sy D, Flouzat C, Eon S, Charlier M, Spotheim-Maurizot M (2001) Modelling radiation-induced damage in the lac operator-lac repressor complex. DNA damage 8-oxoguanine. Theor Chem Acc 106 137-145... 

The lacl gene has its own promoter (Pkd) to which RNA polymerase binds and initiates transcription. In the absence of an inducer, the lacl gene is transcribed, producing lac repressor mRNA and hence lac repressor protein monomers. These monomers assemble to form active tetramers which bind to the lac operator site, 0, , and prevent transcription of the lac operon. In the presence of an inducer (such as allolactose or IPTG), the inducer binds to the repressor and changes its conformation, reducing its affinity for the lac operator. Thus the repressor now dissociates and allows RNA polymerase to transcribe the lac operon. 

If inducer is removed, the lac repressor rapidly binds to the lac operator site and transcription is inhibited almost immediately. The lacZYA RNA transcript is very unstable and so degrades quickly such that further synthesis of the P galac-tosidase, permease and transacetylase ceases. 

Binding of tryptophan to the aporepressor apparently produces a conformational change that enhances the binding to the trp o site. The target for the tryptophan-repressor complex is again a DNA sequence with twofold symmetry. Contrary to lactose, which causes the release fo the repressor from the lac operator, tryptophan acts as a corepressor, stimulating the binding of the repressor to the trp operator. 

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