Different arrangements of protein subunits and single-stranded circular DNA in the filamentous bacterial viruses fd and Pf1

Abstract

A single-stranded circular DNA molecule of 6690 ± 450 nucleotides accounts for 5.5 ± 0.3% of the mass of Pf1 virus. The remaining mass is contributed almost entirely by subunits of the major coat protein. A non-integral nucleotide to subunit ratio of 0.87 ± 0.05 is calculated from the DNA content, the average nucleotide mass (309), and the known mass of one protein subunit (4609). There are therefore 7690 ± 680 major coat protein subunits in the virus. The virus length determined by electron microscopy is 1960 ± 70 nm. The data give an average axial distance of 2.55 ± 0.24 Å between protein subunits in dry virus. Since there is an up strand and a down strand of the circular DNA within the virus filament, an axial distance between bases in a given strand of 5.9 ± 0.5 Å is calculated. Available X-ray data show that an axial repeat of 72 Å, or slightly less, would be expected for dry Pf1 virus (0% relative humidity). A structural model in which 27 protein subunits and 24 nucleotides are contained in this repeat would be consistent with our data. The DNA conformation and the subunit packing in Pf1 differ considerably from those in fd, even though both are filamentous viruses containing single-stranded circular DNA. The uncertainties cited are 95% confidence limits.