A synthetic transcriptional counter

Elementary sequential logic circuits have been proposed in synthetic biology. The long-term goal is to obtain more complex circuits, capable of finite-automaton computation, in analogy to central processing units in digital computers. We design here a single bit counter, i.e. a parity checker, that can be implemented with current technology by a genetic circuit. When excited by a sequence of N sufficiently separated pulses of an external input (which might be a chemical inducer or a physical signal such as light at a specific frequency), the network will produce an output to indicate whether the number N of pulses seen so far is even or odd. Our circuit is the key component of a distributed-computation m-bit counter, in which a count modulo 2^m of the observed number N of pulses is stored and displayed. The m-bit counter can be in principle assembled from a set of m single-bit counters, together with additional gates that implement "carry" operations. In this design, communication among cells could be biologically implemented by diffusing chemicals (for example, quorum-sensing molecules).