Locating the minor components of double-stranded RNA bacteriophage 6 by neutron scattering

The polymerase core of double-stranded (ds) RNA virus provides the molecular machinery for RNA packaging and replication. Procapsid of bacteriophage 6 constitutes the best studied model of such dsRNA-processing machine. A total of 120 copies of protein P1 form the procapsid framework to which other proteins are attached. Due to their low abundance minor procapsid constituents, P2 (RNA polymerase) and P7 (packaging factor), have not up to now been localized. We have applied small-angle neutron scattering (SANS) and contrast variation in order to localize the two proteins. Procapsids containing deuterated P2 or P7 were produced in vitro and SANS was collected at several contrast levels. Radial positions of labeled proteins were obtained and modeled within the electron density of the procapsid. P2 monomers reside at each five-fold vertex just under the RNA packaging complex. P7 was detected at a distance of 160 A from the procapsid center indicating localization on the inner surface of P1 framework.