Bursty Protein Production. Two simulations contrasting the behaviour of protein abundance for a moderate repressor with k
of 1 nM (solid line) and a strong repressor with k
of 0.01 nM (dashed line). For the 1 nM repressor, all other parameters are as in Figure 1(a). For the strong repressor, k
has been adjusted so that the both models have an average protein abundance of 100 molecules per cell – this allows both simulations to be plotted on the same axes so that the noise can be easily compared. In both cases, protein is produced in bursts. With the moderate repressor, the bursts are short and protein level is being continuously adjusted about the mean. With a strong repressor (low k
), the bursts are large and coincident with small number of times in this simulation that mRNA is synthesized. This is the source of the additional variability over and above the linearized system. It is also important to observe that the variability in protein abundance – at least for a stable protein – is slow relative to the cell cycle time. This means that extrinsic noise due to DNA and cell replication are likely to contribute very significantly to strongly auto-repressing transcription factors.