Abstract
Below 15°C, chloroform causes fd phage to contract to I-forms, which are compact structures about 1/3 as long as the original phage. Above 15°C, chloroform causes I-forms to contract to even more compact spheroidal S-forms. Here we show that the coat protein structure in I-forms is the same as the protein structure in the phage and the protein structure in S-forms is the same as the protein structure in bilayers. The conversions from fda˚I-forms→S-forms are therefore suggested to mimic steps in fd penetration. The same conversions, in reverse order, are suggested to mimic steps in fd assembly.