Wrapping of flanking non-operator DNA in lac repressor-operator complexes: implications for DNA looping.

摘要: Abstract In our studies of lac repressor tetramer (T)- operator (O) interactions, we observed that the presence extended regions non-operator DNA flanking a single sequence embedded in plasmid produced large and unusual cooperative anticooperative effects on binding constants ( K obs ) their salt concentration dependences for formation 1:1 (TO) especially 1:2 (TO 2 complexes. To explore origin this striking behavior report analyze data complexes between O sym 40 bp, 101 2514 bp DNA, over very wide ranges [salt]. We find interrelated length [salt] TO , [salt]-derivatives, quantify these terms free energy contributions two wrapping modes, designated local global. Both global occur to an increasing extent as decreases. Global plasmid-length is extraordinarily dependent propose driven at low by polyelectrolyte effect, involves number (≳20) coulombic interactions phosphates positively charged groups repressor. Coulombic wrap must involve both core second DNA-binding domain repressor, result complex which looped wrapping. The non-coulombic contribution highly unfavorable (∼+30-50 kcal mol −1 ), presumably results from significant distortion and/or entropic constraints. structural model wrapping, consider its implications looping intervening forming tetrameric (LacI) one multi-operator molecule vivo vitro . existence LacI-DNA motivates proposal most if not all proteins may have evolved capability thereby organize DNA.