Footprinting DNA-Protein Interactions Powerful and fairly rapid methods for mapping where and how proteins bind tightly to DNA 2 ways: 1.DNAse I footprinting 2.DMS footprinting

1.Prepare end-labeled DNA. 2.Bind protein. 3.Mild digestion with DNAse I (randomly cleaves DS DNA on each strand) 4.Separate DNA fragments on denaturing acrylamide gels. DNAse I Footprinting Fig. 5.37a

Sample of a DNase I footprinting gel. Footprint Samples in lanes 2-4 had increasing amounts of the DNA-binding protein (lambda protein cII); lane 1 had none. Fig. 5.37b

Dimethylsulfate (DMS) Footprinting 1.End-label DNA fragment. 2.Bind protein. 3.Treat with DMS, methylates purines. 4.Partially cleave DNA at the methylated bases. 5.Separate fragments on gel. Fig. 5.38a

Example of DMS footprinting. Lanes 1 and 4 had no protein Lanes 2 and 3 had 2 different amounts of protein. Protein binding protects most purines from modification by DMS, but it can stimulate modification of those in regions where the helix is distorted or partially melted (indicated by *). Fig. 5.38b

Positive Control of Lac Operon Catabolite Repression hypothesis –predicted that glucose would inhibit synthesis of other sugar metabolizing pathway enzymes (e.g., lactose pathway) Partially right, its lack of activation instead of true repression –Cells respond to high glucose with lowered levels of cAMP and vice-versa –cAMP activates Lac operon via CAP

CRP bends --> cyclic 5’-3’ phosphodiester in cAMP - Stimulates Lac operon (lacZ production) as the co-activator for the CAP protein glucosecAMP

CAP (catabolite activator protein), a.k.a. crp (cAMP receptor protein) gene CAP only active bound to cAMP CAP-cAMP stimulates transcription by promoting formation of closed complex: RNAP + Pro ↔ RPc → RPo (RPc = Closed complex) K b k 2 (RPo = Open complex) K b – equilibrium binding constant for formation of RPc k 2 – rate constant for formation of RPo CAP-cAMP increases K b

Lac Control Region Fig CAP binds just upstream of promoter L1 deletion mutant has constitutively low expression

CAP-cAMP dimer interacts with the CTD of the  subunits of the RNAP Core CTD - carboxy-terminal domain NTD - amino-terminal domain Fig CAP-cAMP is a dimer that binds to a short sequence (~20 bp) with dyad symmetry (activator site) αCTD binds DNA too

CAP-cAMP-  CTD and CAP-cAMP-DNA complexes: CAP-cAMP bends the activator DNA Fig 7.17

Why does the Lac Operon need an activator? Not a very good core promoter: TTTACAC TATGTT (Lac) TTGACAT TATAAT (consensus) CAP stimulates more than 100 promoters!

Tryptophan operon: Regulation by attenuation Genes for tryptophan synthesis Repressed by end-product of pathway, Tryptophan Repression requires Operator sequence, Aporepressor (trpR gene product) & Co- repressor (Tryptophan) - Operator is within the promoter Also controlled by attenuation in the “Leader” region of the transcript

Attenuation--> Low [tryptophan], aporepressor doesn’t bind Operator, transcription on! High [tryptophan], repressor (aporep. + tryp.) binds operator, represses transcription!

Transcription stops in the leader-attenuator “L” region when the [tryptophan] is elevated.

The trp Leader peptide (14 aa) has two key tryptophan codons. The ribosome stalls at the trp codons when [tryptophan] is too low. The stalled ribosome prevents a downstream transcription terminator (IR + U-rich sequence) from forming. Fig. 7.31

Fig. 7.32

How is translation of the downstream genes achieved with the leader peptide there to stop the ribosomes? Biological advantage: Repression alone decreases expression 70-fold Repression plus attenuation decreases expression 700-fold