Turning Cro into a Transcriptional Activator Fred Bushman and Mark Ptashne Cell (1988) 54: Presented by Drew Endy and Natalie Kuldell for / February 6th, 2008

Small patch of acidic residues is necessary and sufficient for transcriptional activation Figure 1 cI normally activates transcription cro normally represses transcription cro/cI chimera activates transcription!

4 amino acid substitution --> “ cro67” Figure 2 cartoon of cI binding DNA Thr17 Lys21 Asp22 Tyr26 Site-directed mutagenesis of cro helix to make acidic patch fig from “A Genetic Switch”

Why might this work?

4 amino acid substitution --> “ cro67” Figure 2 Thr17 Lys21 Asp22 Tyr26 Site-directed mutagenesis of cro helix to make acidic patch

4 amino acid substitution --> “ cro67” Figure 2 Thr17 Lys21 Asp22 Tyr26 Site-directed mutagenesis of cro helix to make acidic patch

4 amino acid substitution --> “ cro67” Figure 2 Thr17 Lys21 Asp22 Tyr26 Site-directed mutagenesis of cro helix to make acidic patch

4 amino acid substitution --> “ cro67” Figure 2 Thr17 Lys21 Asp22 Tyr26 Site-directed mutagenesis of cro helix to make acidic patch

Protein  -helix recognizes sequence in DNA major groove model of lac repressor binding lac operator

Protein  -helix recognizes sequence in DNA major groove Wild type cro binds O R 3>>O R 2 = O R 1 binding to O R 3 shuts off tx’n from P RM Wild type cI binds O R 1>O R 2>O R 3 binding to O R 2 activates tx’n from P RM

Protein  -helix recognizes sequence in DNA major groove Wild type cro binds O R 3>>O R 2 = O R 1 binding to O R 3 shuts off tx’n from P RM Wild type cI binds O R 1>O R 2>O R 3 binding to O R 2 activates tx’n from P RM cro67 binds? activates? Figure 3 O R 1>O R 2>O R 3

Protein  -helix recognizes sequence in DNA major groove Wild type cro binds O R 3>>O R 2 = O R 1 binding to O R 3 shuts off tx’n from P RM Wild type cI binds O R 1>O R 2>O R 3 binding to O R 2 activates tx’n from P RM cro67 binds? activates? Figure 3 O R 1=O R 2>O R 3

cro67 activates transcription in vitro In vitro tx’n rxn’s + buffer + DNA w/ P RM + P R  cro67 (purified) + 32 P-ATP, CTP, GTP or UTP 37° 10’ then + RNAP 37° 10’ then +formamide to gel Figure bases 250 bases [ cro67] 0

Observe: txn of P R as txn of P RM when cro67 added cro67 activates transcription in vitro Figure bases 250 bases ~5x cut out bands and count Q’s: What are extra bands? Is cro67 bound in natural way?

Observe: cro67 binds operator sequences as expected Figure 4 Q: is assay sensitive to different conformations of bound prot? DNase footprint O R 1=O R 2>O R 3 [ cro67] 0 + buffer + 32 P-DNA w/ P RM + P R  cro67 (purified) 37° 10’? then + DNase 37° 10’? then +formamide to gel?

cro67 activates transcription in vitro Supporting data/controls Figure 5 Wild type cro does not activate txn in vitro using in vitro txn rxn, DNase ftpt Figure 6 cro67 does not activate txn from other promoters cro67 in vivo exp’ts hampered by low affinity for operators (~100x < wt cro)

look at****** cI vs434 cI patch more acidicinc act’n patch more basicdec act’n operator occupancysat’d operator bindingnormal Summary of 434 cI data ** in vivo (  -gal assays on lysogen) ** in vivo DMS ftpt ** in vitro txn rxns, DNase ftpt

Turning cro into a transcriptional activator key assumption in vitro conclusions have meaning in vivo biggest mistake INKHO: mixing the 434 work in IDEHO: not pushing in vivo work significance/meta-lessons – protein engineering by analogy (cro is like cI, thus…) – small changes (e.g., individual AAs) are important – good data enables thoughtful experiments – be open to surprises (e.g., DNA binding) – ask the next question: does activation work the same way in eukaryotic cells?