We study both in silico and in vivo the real-time feedback control of a molecular titration motif that has been earmarked as a fundamental component of antithetic and multicellular feedback control schemes in E. coli. We show that an external feedback control strategy can successfully regulate the average fluorescence output of a bacterial cell population to a desired constant level in real-time. We also provide in silico evidence that the same strategy can be used to track a time-varying reference signal where the set-point is switched to a different value halfway through the experiment. We use the experimental data to refine and parameterize an in silico model of the motif that can be used as an error computation module in future embedded or multicellular control experiments. the experimental work described in this paper. The authors thank Dr Dan Rocca and Dr Elisa Pedone for support with the microfluidics platform set-up, and Dr Mark Jepson and Alan Leard (Wolfson Imaging Facility, University of Bristol) for supporting live-cell imaging experiments.