A structural model for the single-stranded DNA genome of filamentous bacteriophage Pf1.

The filamentous bacteriophage Pf1, which infects strain PAK of Pseudomonas aeruginosa, is a flexible filament ( approximately 2000 x 6.5 nm) consisting of a covalently closed DNA loop of 7349 nucleotides sheathed by 7350 copies of a 46-residue alpha-helical subunit. The subunit alpha-helices, which are inclined at a small average angle ( approximately 16 degrees ) from the virion axis, are arranged compactly around the DNA core. Orientations of the Pf1 DNA nucleotides with respect to the filament axis are not known. In this work we report and interpret the polarized Raman spectra of oriented Pf1 filaments. We demonstrate that the polarizations of DNA Raman band intensities establish that the nucleotide bases of packaged Pf1 DNA are well ordered within the virion and that the base planes are positioned close to parallel to the filament axis. The present results are combined with a previously proposed projection of the intraviral path of Pf1 DNA [Liu, D. J., and Day, L. A. (1994) Science 265, 671-674] to develop a novel molecular model for the Pf1 assembly.