A unique DNA curvature, the CIT, has been found in the 5′-upstream region of the psbA2 gene, which exhibits basal, light-responsive and circadian rhythmic transcription, in a unicellular photosynthetic cyanobacterium, Microcystis aeruginosa K-81. In this study, we report the universality of curvatures found in 5′-upstream regions in the psbA family and the function of the curvature in gene expression. Intrinsic curvatures were identified within 1000 bp upstream from the psbA genes in another cyanobacterium, a red alga and in plants (monocot and dicot). Mutagenized curvatures were constructed and confirmed to have disrupted architecture by gel electrophoresis and atomic force microscopy. Relatively small amounts but light-responsive transcripts of psbA2 were observed in cyanobacterial transformants harboring the mutagenized curvature under light/dark and light/high-light conditions. This shows that the curvature is important for basal transcription. In vitro primer extension and DNA mobility shift assay revealed that factors which might bind to the region upstream from the bending center contribute to the effective basal transcription of psbA2.
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