Designing biological systems: Systems Engineering meets Synthetic Biology

Synthetic Biology offers qualitatively new perspectives on the benefits of industrially harnessed biological processes. The ability to modify and reprogramme natural biology increases the scope of tailored bioprocesses and yields attractive prospects beyond conventional Biotechnology. The present review summarises the major achievements and categorises them according to a hierarchy of system levels. Similar structures are known in the engineering sciences and might prove useful for the future development of Synthetic Biology. The hierarchy encompasses several levels of detail. Biological (macro-)molecules present the most detailed level (parts), followed by compartmentalised or non-compartmentalised modules (devices). In the next level, parts and devices are combined into functional cells and further into cellular communities. The manifold interactions between biological entities of the same hierarchical level or between different levels are accounted for by networks, primarily metabolic pathways and regulatory circuits. Networks of different types are represented as a superordinate hierarchical level that achieves full system integration. On all these levels, extensive and sound scientific foundations exist regarding experimental but also theoretical methods. These have led to diverse manifestations of Synthetic Biology on the parts and devices levels. Investigations involving synthetic components on the systems scale represent the most difficult and remain limited in number. A main challenge lies with the quantitative prediction of interactions between different entities across different scales. Systems-theoretical approaches provide important tools to analyse complex biological behaviour and can support the design of artificial biological systems. A promising strategy is seen in an efficient modularisation that reduces biological systems to a limited set of functional modules with well-characterised interfaces. For the design of synthetic biological systems the interactions across these interfaces should be standardised to reduce complexity. Yet, the identification of modules and standardised interaction routes remains a non-trivial problem. Furthermore, an appropriate platform that efficiently describes replication and evolutionary processes has to be developed in order to extend the achievements of Synthetic Biology into designed biological processes.