A duplex structure involving two non-complementary DNA strands can be formed and stabilized by M13 phage proteins

M13 phage is a long, thin nucleoprotein filament containing a single-stranded DNA loop. Exposing the filaments to a chloroform/water interface at 20 °C causes them to contract into hollow spherical particles (spheroids), while exposure at low temperatures yields short, thick rods (I-forms). All of the DNA remains within the I-forms while a specific third remains within the spheroids. Here, a photo-crosslinking reagent, psoralen, has been used to probe secondary structure of the DNA in situ in these chloroform-relaxed phage forms. Following photo-crosslinking, the DNA that had been held within the spheroids appeared to be a duplex rod when seen by electron microscopy, while the DNA extruded from the spheroids was an open single-stranded DNA loop. Photo-crosslinking of the DNA in the I-forms yielded linear duplex DNA rods close to the length of M13 phage filaments. Similar observations derived from experiments with deletion and insertion mutant phage showed that the stability of the duplex rods did not depend on the sequence homology between the two opposing strands. These results showthat two non-homologous strands of DNA can exist in an apparently duplex structure and suggest that this is directed by proteins, possibly involving interaction at a membrane.