  The lambda phage is a paradigm temperate bacteriophage. Its lysogenic and
lytic life cycles echo competition between the DNA binding CI and CRO proteins.
Here we address the Physics of this transition in terms of an energy function
that portrays the backbone as a multi-soliton configuration. The precision of
the individual solitons far exceeds the B-factor accuracy of the experimentally
determined protein conformations giving us confidence to conclude that three of
the four loops are each composites of two closely located solitons. The only
exception is the repressive DNA binding turn, it is the sole single soliton
configuration of the backbone. When we compare the solitons with the Protein
Data Bank we find that the one preceding the DNA recognition helix is unique to
the CI protein, prompting us to conclude that the lysogenic to lytic transition
is due to a saddle-node bifurcation involving a soliton-antisoliton
annihilation that removes the first loop.
