Brewster, R.C. et al. Cell 156, 1312–1323 (2014).

When predicting the regulatory effect of a transcription factor (TF) in bacterial gene expression, many models assume the TF to be in excess of its binding sites on a promoter. But as Brewster et al. recently pointed out, this assumption is not always valid, particularly in cases where genes are expressed at high copy numbers or where many genes compete for a TF. The researchers describe a simple regulatory circuit in which a TF represses a reporter to measure the relationship between TF and various reporter copy numbers in the absence or presence of competing TF-binding sites. The resulting model will help fine-tune the output of a genetic circuit without requiring modification of the regulated gene.