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As indicated earlier, in order for the modified SMB to behave as a secure source of sterile somatic cells, 2 events should be initiated upon activation of a switch. This could be done in the following manner:

• EVENT 1) In order to achieve massive induction of recombination, the SMB could include a “dormant” plasmid born cre gene. This spare copy of cre would be placed under the control of an inducible promoter with low basal expression. Upon demand, Cre could be over expressed, leading to high levels of lox recombination. Thus leading to massive G to S differentiation. A certain proportion of G cells would however persist.

• EVENT 2)The solution to second challenge could be coupling the high recombination switch to a retarded suicide switch in the gem line cells. This could be the expression of ccdB suicide gene in the germ line cells.

This could be the opportunity to practically apply one of the founder principles of synthetic biology: modularity in systems assembly.

Indeed, a time delay circuit has already been described, constructed & theoretically studied (Sara Hooshangi et al. 2005, Sara Hooshangi et al. 2006 and Juan M. Pedraza et al. 2006). Using such a time delay circuit could allow using a single signal to induce event 1, followed after a delay by event 2.

Ref.

• Ultrasensitivity and noise propagation in a synthetic transcriptional cascade

Sara Hooshangi, Stephan Thiberge, and Ron Weiss PNAS � March 8, 2005 � vol. 102 � no. 10 � 3581–3586

• Noise Propagation in Gene Networks

Juan M. Pedraza and Alexander van Oudenaarden SCIENCE VOL 307 25 MARCH 2005

• The effect of negative feedback on noise propagation in transcriptional gene networks

Sara Hooshangi and Ron Weiss CHAOS 16, 026108 (2006)