Pf1 bacteriophage replication--assembly complex. X-ray fibre diffraction of the high humidity form

The helical intracellular nucleoprotein complex of Pf1 bacteriophage has been studied by X-ray fibre diffraction in various hydration states. The helix pitch changes from 44 A in dry fibres to 55 A in wet fibres, whereas the unit rise between subunits in the helix apparently does not change with humidity. This result indicates that the nucleoprotein assembly twists more readily than it stretches. This is consistent with its biological role of tightening the viral DNA into a more compact form for packaging in the virion.