Dynamics of RpaB-promoter interaction during high light stress, revealed by chromatin immunoprecipitation (ChIP) analysis in Synechococcus elongatus PCC 7942

In cyanobacteria, a series of genes are induced by, and cause tolerance to, high light stress conditions. Some of these genes share a short, repeated sequence motif known as a high light regulatory 1 (HLR1) element in their promoter regions. Previously, RpaB, a two-component response regulator, was shown to interact with the HLR1 element of several high light-responsive promoters in vitro. However, how RpaB regulates target promoters in vivo remained elusive. In this study, we analyzed the role of RpaB in transcriptional regulation of high light-responsive genes by chromatin immunoprecipitation (ChIP) analysis, which has been recently developed and utilized to study in vivo interactions between DNA-binding proteins and the relevant target DNA. One of the advantages of this method is the ability to detect dynamic interaction patterns in response to various growth and/or environmental conditions instantaneously at the time of the analysis. Here we examined the binding patterns of RpaB under various light conditions using ChIP assays. We found that strong interactions of RpaB with target promoters were weakened in a high light-dependent manner, and that the lower binding level of RpaB continued as long as the high light conditions were maintained. Thus, in regulation of high light-inducible genes, we suggest that RpaB functions as a repressor under normal light conditions, and that high light conditions result in release of the repression.