The precursor complex of Pf3 bacteriophage

Filamentous bacteriophage infections give rise to intracellular filamentous precursor complexes composed of a circular single-stranded DNA molecule and on the order of 103 copies of a viral-encoded, single-stranded DNA binding protein. A protocol was developed for the purification of the precursor complex from Pf3 phage-infected Pseudomonas aeruginosa cells, and existing protocols for Ff and Pfl complexes were amended by the introduction of a molecular sieving column step. The Pf3 precursor complex has an average contour length of 500 nm, which is shorter than that of the mature Pf3 virus. M/L (mass/ length) values were obtained from turbidity measurements, from scanning-transmission electron microscopy, and from the total particle mass divided by its length. The average M/L obtained was 19,300 Da/nm which is close to that of the Pf3 virion. The complex has a nucleotide/subunit ratio of 6.0. The protein component of the precursor complex has less than 10% α-helicity, whereas the protein component of the virus is greater than 90% α-helical. The DNA structure in the precursors is very different from that in the virus, so that during virus assembly the DNA structure must change dramatically. The results for the Pf3 system, together with the available information for the Ff and Pfl filamentous virus systems, indicate that different DNA-protein interactions and packing arrangements are involved in performing equivalent functions.