Filamentous bacteriophage contract into hollow spherical particles upon exposure to a chloroform-water interface.

The bacteriophage M13 is a 1 micrometer long filament consisting of a circular single-stranded DNA loop firmly held within a tubular protein and capsid. We report here that exposure to a chloroform-water interface initiates a 20 fold contraction of each filament into a hollow protein sphere. In these 0.04 micrometer diameter particles, termed M13 "spheroids," two thirds of the DNA is apparently extruded through a hole in the wall of the spheroid; the portion of DNA remaining inside the shell centers about the origins of M13 DNA replication. These results suggest that the filament, upon exposure to a membrane environment, undergoes an ordered change whereby the DNA is released into the cell and the coat protein is changed to a form more easily solubilized by the membrane lipids.