Only depends on B. The only mutual influence between TFs is that B associates and dissociates more slowly when A is bound to the promoter. We used realistic parameter values similar to those reported by quantitative thermodynamics [79,89] and 3-MA supplier single-molecule kinetics studies [90] of bacterial regulation for concentrations ([A] ?[10-2; 103] nM and [B] = 5 nM), for bimolecular TF-DNA residence timesCoulon et al. BMC Systems Biology 2010, 4:2 http://www.biomedcentral.com/1752-0509/4/Page 12 ofoff (1/ k off = 30 s and 1/ k B = 60 s) and equilibrium conA d = 0.5 nM and d stants ( K A K B = 5 nM) and for modification of activation energy upon interaction (E = 2.5 kcal.mol -1 ). RNA and protein life-times are 5 and 20 min [91]. All parameters are summarized in table S2 of Additional file 1. This simple system demonstrates not only that a TF can regulate the variance of RNA and protein levels without influencing their mean (as previously identified with various mechanisms [28,42]), but more interestingly, that normalized variance can increase with the concentration of a TF (figure 3B). Indeed, here it is at high concentrations of A that events influencing transcription (ie. B associations and dissociations) become rare and, as it has been known for a long time [2,3,14,23-27,30,32,34-37,40], slow promoter dynamics result in strong variability. RNA distribution goes from unimodal to bimodal as [A] increases (figure 3C). Thus, the oversimplistic assumption that increasing the concentration of a TF necessarily reduces the stochasticity is not always valid. Moreover, this property appears to be quite robust when exploring all sorts of deviations from this ideal case (eg. considering a different concentration for [B], an influence of B on A, a dependency of the transcription rate on A …, figure S1 in Additional file 1), and even for a more complex regulatory system with a larger number of TFs and randomly drawn parameters (figure S2 in Additional file 1). Simple PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/28993237 molecular scenarios can be imagined that would give rise to this behavior. For instance, the shape of a TF (type A) can be so that, when bound, it prevents other TFs (type B) from association/dissociating from the promoter (figure 3A2). Another example would be to consider B the chromatin state and A a TF that binds to the same site as chromatin remodeling complexes. Then, designing molecular constructions to verify this hypothesis experimentally appears to be promising.argue in Discussion that several typical features of eukaryotic promoter are precisely those that give rise to a complex dynamics. We also show how this complexity is hidden by common measures and/or modeling frameworks and identify what kind of features can modulate or constrain it.Steady-state aspectsPotentiality of the molecular interplayUsing this generic model and the prediction of the different indicators we provided allows us to explore in detail the activity of regulatory systems on large ranges of concentration. To illustrate the potentiality of steadystate and dynamic properties, we consider here two examples (figures 4 and 5) dedicated to represent respectively a prokaryotic promoter – with typical features as in [56,67,80] – and a eukaryotic promoter (the same as in figure 2) – reproducing the periodic behavior as observed in [48-52,66]. Although it is the expected behavior that complex activities can arise from large systems with many parameters, we show that such behavior can occur with physically realistic par.