A naturally occurring genetic switch which controls the
lytic/lysogenous switch in bacterial cells infected with the 
phage has been extensively studied. Ptashne's classic book on the
subject is required reading to instill modesty among those who would
engineer these systems.
While the details of the switch mechanism are (typically)
substantially more complex than the techniques proposed here, it is
striking that both the CRO and
repressor are dimeric
protein complexes, and that both interact cooperatively with gene
control sites.