We are studying protein-nucleic acid interactions that occur in the assembly pathways and the final structures of several filamentous bacterial viruses, all of which are in the genus Inovirus. At the ends of the viruses, where the DNA molecules fold back on themselves, there are a few copies each of different minor proteins. The viruses are assembled at the cell membranes of the host cells in extrusion processes that do not lyse the cells. Given the variety of different species, the genus provides many instances in which exactly the same function is carried out by molecules of diverse sequence, allowing fruitful comparative analyses. One such instance is the way DNA is arranged in the different virions, the function being the protection of the DNA in a structure that can be efficiently assembled by the cell, yet efficiently disassembled when the virus infects a new cell.
Dramatic differences occur in the details.
The structural unit of two subunits and two nucleotides, one from each strand, has a rise of 6 A and a rotation of 1320, values which are about two and four times those for classical B-DNA, respectively. The protein coat forces the phosphates of both strands to the center and holds the bases unstacked on the outside. Integer stoichiometry in C2 would allow its protein shell and its DNA to follow exactly the same helical symmetry rule, as is true for Pf1, but whether or not they do in C2 is not yet known. It is clear from this brief comparison that fundamental issues of the chemical natures of DNA-protein interfaces can be addressed through comparative studies of species within this genus of viruses. An electrostatic spatial resonance model for coaxial helical structures with applications to filamentous bacteriophages.