a, The intracellular negative-feedback (NFB) oscillator is a synthetic module that works at a single-cell level in bacteria. The module produces λ-repressor protein (purple), which inhibits the oscillator's own activity, and a cyan fluorescent protein (blue). These proteins are degraded by the ClpXP protease enzyme, resulting in high-frequency, low-amplitude oscillations, measured by levels of cyan fluorescence (depicted by blue lines). b, A quorum-sensing (QS) module produces the negative feedback protein AiiA (green) and a yellow fluorescent protein (yellow), which are also degraded by ClpXP. Oscillation of the QS module is synchronized throughout the microbial population, and oscillations occur at low frequency and high amplitude (depicted by yellow lines). c, Prindle et al.2 have generated the first complex synthetic biocircuit by simultaneously introducing these two oscillators into Escherichia coli cells. When both compete for degradation by ClpXP, their dynamics are synchronized.