MATCHING ELECTROSTATIC CHARGE BETWEEN DNA AND COAT PROTEIN IN FILAMENTOUS BACTERIOPHAGE - FIBER DIFFRACTION OF CHARGE-DELETION MUTANTS

The virion of Ff (fd, fl, M13) filamentous bacteriophage consists of a long tube of coat protein subunits in a shingled, helical array, surr ounding a genome of circular single-stranded DNA. Modified fd virions have been generated by a mutation (K48A) that removes one positive cha rge from each coat protein subunit in the C-terminal region of the pol ypeptide chain facing the DNA. The number of nucleotides in the mutant DNA is unchanged, but the K48A virions are 35% longer than wild-type. We have measured the X-ray diffraction attributable to single virions in hydrated gels of wild-type and K48A bacteriophages. Most of the di ffraction pattern shows no significant difference between wild-type an d K48A. Since the DNA is only about 12% by weight of the wild-type vir ion, the diffraction pattern is dominated by the protein contribution, and the absence of significant differences indicates that there are n o significant changes in the symmetry or structure of the protein coat . But there is a change in the diffraction pattern in a region where t he DNA and protein contributions are comparable. The diffraction patte rn of the K48A mutant shows an increase in intensity of one of the wea ker equatorial peaks, relative to wild-type, in a region where the pro tein contribution has negative sign but the DNA contribution has posit ive sign. This is consistent with a decrease in the ratio of DNA:prote in per unit length of the K48A mutant. The results support the view th at the protein forms a sheath lined with positive charges interacting electrostatically and non-specifically with a negatively charged DNA c ore of matching charge density The lower positive charge density Linin g the capsid in the K48A mutant means that correspondingly fewer nucle otides can be packaged per coat protein subunit, which in turn require s an elongation of the DNA inside the virion. A longer virion is thus required to package the same amount of DNA. Within the error of measur ement, the number of positive charges on the protein interacting with the DNA is the same in K48A as in the wild-type, despite the fact that the mutant is 35% longer than the wild-type.