Cell-free transcription-translation provides a simplified prototyping environment to rapidly design and study synthetic networks. Despite the presence of a well characterised toolbox of genetic elements, examples of genetic networks that exhibit complex temporal behaviour are scarce. Here, we present a genetic oscillator implemented in an E.coli based cell-free system under steady-state conditions using microfluidic flow reactors. The oscillator has an activator-repressor motif which utilizes the native transcriptional machinery of E.coli; the RNAP and its associated sigma factors. We optimized a kinetic model with experimental data using an evolutionary algorithm to quantify the key regulatory model parameters. The functional modulation of the RNAP was investigated by coupling two oscillators driven by competing sigma factors, allowing the modification of network properties by means of passive transcriptional regulation.