Riboswitches are regulatory elements residing in the untranslated regions of mRNA that control translation through direct ligand

binding. The advantage of riboswitches is that they are much simpler to engineer than proteins. Of the systems described above, the arabinose sensing [ 37] and the theophylline sensing [ 38•] systems were reconstituted in phospholipid vesicles, thus allowing for the development of cellular mimics capable of responding to the chemical composition of their extravesicular surroundings. Non-genetically encoded sensing mechanisms are a potential complement to the use of protein and RNA sensors. The aqueous two phase system developed by Keating and colleagues can be used to MG-132 cell line control the localization of molecules in response to environmental fluctuations. This is because many biological molecules undergo structural changes that affect their surface charge distribution upon shifts in pH or temperature [39•]. Sensing that results in the movement of a chemical system is also possible [40] (Figure 3b). Hanczyc and colleagues built a chemical system that moves away from depleted nutrients and towards molecules that sustain movement. Now that it possible

to build cellular mimics that sense and respond to changing chemical conditions, it seems that the time is right to begin to more deeply probe non-replication aspects of life. Sensory pathways SAHA HDAC are required for the Chlormezanone construction of systems that better represent the complexities of extant life. Unlike

life, machines are programmed to act in a very defined manner, performing a designated task regardless of external conditions. Cellular mimics with sense–response capabilities, therefore, probably would come closer to being perceived as living than a machine. Further, the incorporation of sense–response pathways allows for a more objective means of evaluating success through the implementation of a cellular Turing test. Many of the features of cellular life now can be built in the laboratory. However, the individually reconstituted features of life may not be compatible with each other in their present form. Their integration into a system that better represents the complexity of life poses a significant challenge. It may be that the purely chemical approaches and those that make use of biological molecules will continue to proceed on separate tracks, which would be unfortunate. DNA replication is easier to achieve with the aid of proteins and vesicle division is simpler through purely chemical–physical means. If these two branches of bottom-up synthetic biology found a way to merge, perhaps the synthesis of an artificial cell would be much nearer.