Structural parameters of the Pf1 gene 5 protein-DNA complex in solution by neutron scattering

Neutron-scattering experiments have been performed on the intracellular complex formed by the gene 5 protein and single-stranded DNA in cells infected by filamentous bacteriophage Pf1. The contrast matched point of the complex (37% 2H2O) is lower than expected and implies that a substantial fraction of potentially labile hydrogen atoms are unable to exchange with the solvent. The mass/length ratio of the complex (3270 daltons/A) indicates an axial subunit repeat of 5.1 A, a value much larger than the subunit repeat previously determined in fibres. The measured value of the cross-sectional radius of gyration at infinite contrast (Rc = 43.3 A) indicates an outer radius of 60 to 63 A for the complex. The variation in Rc with contrast shows that regions of higher scattering density are located, on average, towards the outside of the complex. The high-angle region of the intensity curve (measured in 2H2O) reveals a clear subsidiary maximum at 0.105 A-1 arising from the 60 A helical pitch of the nucleoprotein complex. The structural parameters of the Pf1 gene 5 protein-DNA complex in solution are compared with those of the fd gene 5 protein-DNA complex.