A model of lambda DNA arrangement in the viral particle.

The icosahedral shape of the lambda head suggests a 12-subunit structure of the collapsed DNA inside. The internal space of an icosahedron can optimally be filled by 12 geometrical figures each of which is a combination of a cone and more than half of a sphere. Such a pear-like geometrical figure is, in fact, formed spontaneously by DNA collapsed under certain conditions in vitro (Eickbush & Moudrianakis, 1978). It is proposed that a pear-like structure formed by about 4000 bp is the fundamental structural subunit of packaged lambda DNA. A possible arrangement of the 12 subunits inside the phage head relative to the tail is discussed. We hypothesize that lambda DNA is packaged into proheads in its condensed form. A driving force promoting the DNA translocation could be an ATP-dependent activity of a DNA topoisomerase (gpA/gpNu1), which would induce further reduction in the linking number of the already strongly negatively supercoiled DNA by rotation of one DNA strand around the other. The additional strain accumulated at the end of DNA molecule bound by the topoisomerase beyond a critical value would lead to regional collapse of the viral genome into a pear-like structure.