DNA Recognition in Procaryotes by Helix-Turn-Helix Motifs

Lysogeny vs. Lysis

Lysogenic Lytic

Cro from phage-monomer

Cro from phage-dimer

Repressor from phage-monomer

Repressor from phage-dimer

helix-turn-helix

Cro - DNA recognition helices vs. 34 Å

Genetics + Structural Biology

Repressor from 434 phage-dimer

10.2 Most bacterial repressors are dimers containing  helices that insert into adjacent major grooves of operator DNA Figure Copyright (c) by W.H.Freeman and Company

434 repressor-DNA binding BeforeAfter

434 cro/repressor-DNA binding

434 repressor-DNA binding

434 repressor-DNA binding- specific interaction

434 repressor-DNA binding- Non-specific interaction

Protein-DNA interaction in Helix-turn-helix

By itself, an operon is on and RNA polymerase can bind to the promoter and transcribe the genes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig a

However, if a repressor protein, a product of a regulatory gene, binds to the operator, it can prevent transcription of the operon’s genes. –Each repressor protein recognizes and binds only to the operator of a certain operon. –Regulatory genes are transcribed at low rates continuously. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig b

trp repressor-monomer

trp repressor-dimer

trp repressor-DNA

10.2 Ligand-induced conformational changes alter affinity of many repressors for DNA Figure Tryptophan binding induces a conformational change in the trp aporepressor Copyright (c) by W.H.Freeman and Company

10.2 DNase I footprinting assays identify protein-DNA interactions Figure 10-6 Copyright (c) by W.H.Freeman and Company

10.2 Gel-shift assays identify protein- DNA interactions Figure 10-7 Copyright (c) by W.H.Freeman and Company

10.2 The footprint of RNA polymerase and lac repressor on the lac control region Figure 10-8 Copyright (c) by W.H.Freeman and Company

10.2 The lac control region contains three critical cis-acting sites Figure 10-9 Copyright (c) by W.H.Freeman and Company

10.2 Positive control of the lac operon is exerted by cAMP-CAP Figure CAP = catabolite activator protein Copyright (c) by W.H.Freeman and Company

lac repressor-monomer

lac repressor(tetramer)-DNA

CAP-DNA

10.2 A space-filling model of cAMP- CAP bound to lac promoter DNA Figure Copyright (c) by W.H.Freeman and Company

10.2 Cooperative binding of cAMP-CAP and RNA polymerase to the lac contol region activates transcription Figure Copyright (c) by W.H.Freeman and Company