A nucleoprotein complex in bacteria infected with Pf1 filamentous virus: Identification and electron microscopic analysis

We report the discovery, partial purification, and high-resolution electron microscopic characterization of an intracellular complex from Pseudomonas aeruginosa bacteria infected by Pf1 filamentous virus. The Pf1 complex resembles the virion precursor complex of DNA and viral gene 5 protein formed by fd virus of Escherichia coli, but the two complexes differ in structure. Image reconstruction indicates that both complexes are single-start morphological helices; specimen tilting shows the Pf1 helix to be right-handed. Although the Pf1 and fd complexes contain a similar number of nucleotides per axial unit length, the mean distance between helical turns is 61 Å for Pf1 but 91 Å for fd under the conditions used for our measurements; two turns of the fd nucleoprotein helix contain about as many nucleotides as do three turns of the Pf1 helix. The Pf1 complex is much shorter than are Pf1 virions, in contrast to the similar lengths of the fd virion and complex. The fd complex is extremely flexible, but the Pf1 complex is more highly regular in structure. Most significant, calculations based on our data indicate that the DNA in the Pf1 complex is probably located at a smaller radius than in the bulk of the protein. If the DNA and morphological helices coincide, the DNA in the Pf1 complex must be well inside of (axial to) the outeer protein surfaces of the complex, rather than being wrapped around the protein subunits as proposed by others for fd complex.