STRUCTURES OF FD GENE-5 PROTEIN NUCLEIC-ACID COMPLEXES - A COMBINED SOLUTION SCATTERING AND ELECTRON-MICROSCOPY STUDY

Small-angle scattering and electron microscopy studies of fd gene 5 pr otein (g5p) and reconstituted g5p . nucleic acid complexes have been u sed to test models for the complexes and evaluate their uniqueness. In addition, we have obtained new information on the dependence of nucle otide type and protein/nucleotide (P/N) ratio on the structure of the complexes. Reconstituted complexes were made with single-stranded fd v iral DNA (fd ssDNA), poly[d(A)] and poly[r(A)]. All complexes form sim ilar left-handed, flexible superhelices having approximately the same diameter, but the pitch differs among these complexes. The g5p protein is a dimer in solution and the dimers associate to form a superhelica l framework to which the polynucleotide is attached. The combined X-ra y and neutron scattering data confirm the nucleic acid is inside the p rotein superhelix. A Monte Carlo integration modeling procedure applie d to the scattering data was used to systematically test large numbers of possible models for each complex, and previously proposed models b ased on parameters obtained from electron microscopy were found to be essentially correct and unique. The data on the complexes with differe nt P/N ratios showed that mass per unit length values decreased while the rise per dimer and pitch of the superhelix increased for g5p . fd- ssDNA complexes with decreasing P/N ratios.