Overexpression , Purification , and Characterization of Recombinant T 4 Gene 32 Protein 22 , 301 ( g 32 PB ) *

Gene 32 protein (g32P), the replication accessory protein from bacteriophage T4, is a zinc metalloprotein which binds with high cooperativity to single-stranded (ss) nucleic acids. The basic N-terminal 2 1 amino acids (termed the “B” domain) is required for highly cooperative (W = 500) binding of g32P monomers to ss nucleic acids. As part of our studies to systematically evaluate the structural features of the B domain important for cooperative binding, a homogeneous source of g32P which binds noncooperatively to nucleic acids (Cd = 1) and is devoid of contamination by native g32P is needed. Herein, we describe large-scale overexpression and purification of recombinant g32P lacking the tryptic N-terminal B domain (residues l-21), designated g32P-B, as well as its physiochemical and nucleic acid binding properties. G32P-B is readily purified from the soluble fraction of Escherichia coli BL21(DE3) transformed with the plasmid pT7g32B.wt which contains the g32P-B coding sequences under inducible transcriptional control of T7 RNA polymerase. Anion exchange, ssDNA-cellulose and phenyl-Sepharose chromatographies give rise to highly homogeneous g32P-B, free of contaminating nucleic acid. Recombinant g32P-B has the expected Nterminal primary structure and contains stoichiometric Zn(I1). It also has the expected globular structure as shown by ‘H NMR spectroscopy, hydrodynamic measurements, and the ability to selectively remove the carboxyl-terminal “A” domain to form the trypsinresistant g32P-(A + B) DNA-binding core fragment. Quantitative ss nucleic acid binding experiments of g32P-B to poly(dT) (0.05 M NaCl, pH 8.1,20 “C) show that all equilibrium binding isotherms can be fit with W= 1 and&&= 5.2 (21.6) x 10’ M-‘, with a moderate electrostatic component to the binding free energy, 8 log K,,&3 log[NaCl] = -3.0 f 0.2. Under identical solution conditions, g32P-(A + B) derived from g32PB binds to poly(dT) noncooperatively as expected, but with an =&O-fold higher apparent affinity, Kob. = 4.0 (22.0) x 10’ M-l, and detectably enhanced salt sensitivity, a log &,$8 log[NaCl] = -3.9 k 0.3. As the salt concentration is raised, the relative difference in KObs between the g32P-(A + B) and g32P-B is gradually reduced such that extrapolation of the log-log plots to 1 M Na+ standard state gives similar &,. within experimental error. Qualitatively similar observations are also found upon binding to the ribohomopolymer,