Characterizing cross-talk in vivo avoiding pitfalls and overinterpretation

Cross-talk between noncognate histidine kinases and response regulators has been widely reported in vitro and, in specific mutant backgrounds and conditions, in vivo. However, in most cases there is little evidence supporting a physiological role of cross-talk. Indeed, histidine kinases and response regulators show remarkable specificity for their cognate partners. In vivo studies of cross-talk have the potential to establish mechanisms that control specificity and, if the cross-talk is observable in wild-type strains, may reveal new levels of cross-regulation. However such studies can be complicated by effects of other regulatory circuits and by the inactivation of mechanisms that would otherwise suppress cross-talk. It is thus easy to mis- or overinterpret the significance of such studies. We address potential complications associated with measuring cross-talk and discuss some methods for identifying and unmasking sources of cross-talk in cells using transcriptional reporters and in vivo DNA-binding assays.