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As indicated earlier, in order for the modified SMB to behave as a secure source of sterile somatic cells, two 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,, we could add inducible promoter in regulation of Cre expression (pBad for example). It could allow us to differentiate every cell into somatic cell. The generation of a substantial number of somatic cells should be possible if we let the system work alone (positive feedback by pDAPA). And once this number is enough we could add arabinose to express Cre in every germ cell (long and strong induction), leading to massive G to S differentiation. But 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.

In addition Size based screening could be perfomed before envoronmental release. Indeed, S cells having lost FtsK expression present an interresting phenotype: they grow in size without division taking place. The size threshold used in the cell sorting process should be determined by studying size distribution within germ cell and somatic cell populations. This could to improve security.

References

• Ultrasensitivity and noise propagation in a synthetic transcriptional cascade

Sara Hooshangi, Stephan Thiberge, and Ron Weiss PNAS March 8, 2005 vol.102 no. 10 p3581–3586

• Noise Propagation in Gene Networks

Juan M. Pedraza and Alexander van Oudenaarden SCIENCE March 25, 2005 vol.307

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

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