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Abstract

Disaggregated fd coat protein combines under appropriate conditions of pH and temperature with fd DNA and other kinds of single-stranded DNA, but not with double-stranded DNA. Complexes formed with fd DNA are of two types, dependent on whether or not a fragment of the original viral coat persists on the DNA. A 3% fragment of the original coat is sufficient to allow complementation of the DNA by added protein to give an infectious particle indistinguishable from virus. Infectious yields are low, however, and correspond to the completion of only one out of 107 DNA-protein complexes originally present. Complexes formed with a DNA completely stripped of protein are termed irregular. Such complexes differ from phage by lack of infectivity, a lower ratio, higher sedimentation coefficient and a coiled appearance in the electron microscope. The results are interpreted to mean that in the cell-free system used, fd protein is incapable of combining with DNA to give the regular viral pattern, but that a fragment of the viral capsid can serve as a nucleus for the re-formation of the regular structure.