Neutron scattering data on reconstituted complexes of fd deoxyribonucleic acid and gene 5 protein show that the deoxyribonucleic acid is near the center.

We have performed low-angle neutron scattering studies on reconstituted complexes of fd DNA and the gene 5 protein that is produced during infection of Escherichia coli by filamentous fd phage. Essentially identical helical complexes have been made with normal protonated DNA or DNA in which at least 87% of the nonexchangeable protons are replaced by deuterium. From neutron scattering profiles of both complexes over a range of D2O/H2O solvent mixtures, the DNA deuteration is shown to have a dramatic influence on the measured cross-sectional radius of gyration. Most importantly, data for the complex containing deuterated DNA lead to a more negative slope in a plot of the square of the cross-sectional radius of gyration vs. the inverse of the solute-solvent contrast, compared with the slope of a plot of data for the complex containing protonated DNA. This means that, in a cross-sectional view of the complex, the DNA is near the center of the structure. By our analysis, the DNA has a cross-sectional radius of gyration of 17.6 +/- 3 A, while the protein has a cross-sectional radius of gyration of about 33.5 A. Therefore, the model for the structure of the helical complex that has been proposed from X-ray diffraction studies on gene 5 protein crystallized with oligodeoxynucleotides [McPherson, A., Jurnak, F., Wang, A., Kolpak, F., Rich, A., Molineux, I., & Fitzgerald, P. (1980) Biophys. J. 32, 155-170] is not valid for the complex in solution. From our neutron diffraction data we have also obtained values for the solvent-excluded volume and mass per unit length. The relation of our findings to the solution structure of the complex is discussed.