Cyanobacteria (blue-green algae) are photosynthesizing organisms that can be used as a model for analyzing light-responsive gene expression. The regulatory system of the light-responsive psbA gene with cis-elements and trans-acting factors was studied at both transcriptional and post-transcriptional levels. Positive regulation comprises DNA curvatures (CIT and RIB), upstream elements (UPE and promoter), and a light-induced sigma factor (SigD) of RNA polymerase in transcription. On the other hand, negative regulation involves mRNA instability through an AU-box under darkness. This two-step process is a candidate for a novel mechanism regulating light-responsive gene expression.

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