Bacteriophage fd is a class I filamentous virus (others are M13 and fl) that comprises a circular, single-stranded DNA molecule enclosed in a cylindrical protein sheath to form a flexible particle -890 nm long and 7 nm in diameter (for reviews, see refs 1 and 2). The viral DNA contains 6,408 nucleotidesJ.-5 incorporating 10 genes, and the protein sheath is composed of about 2,700 major coat protein subunits6 in a shingled helical array, the symmetry of which is defined by a fivefold rotational axis combined with a twofold screw axis of pitch 3.2 nm (refs 7-9). The DNA extends throughout the length of the particle but is not base-paired and has a symmetry different from that of the protein helix. How the DNA is packed remains unclear but the number (2.4) of nucleotides packaged per major coat protein subunit is certainly not integral, in contrast with, say, the packaging of RNA in tobacco mosaic virus. The coat protein subunit is SO amino-acid residues in length and, in the virus particle, adopts a largely a-helical conformation, with the long axis of the helix aligned close to the long axis of the filament'-9·n. This protein is arranged with its negatively charged N-terminal region on the outside of the filament and its positively charged C-terminal region on the inside abutting the DNA7. We report here that positive charge on one of the four lysine side chains in the latter region has a direct effect on DNA packaging, because when this charge is absent, elongated particles are produced with lengths that can be correlated with the residual positive charge in the C-terminal region of the coat protein subunit.peer-reviewe
