Light-responsive psbA transcription requires the -35 hexamer in the promoter and its proximal upstream element, UPE, in cyanobacteria.

We characterized the function of the -35 hexamer in the promoter and an element just upstream, UPE, in the expression in a unicellular cyanobacterium, Microcystis aeruginosa K-81, of the light-responsive gene psbA2, which encodes a reaction center key protein for photosynthesis. A series of mutants with mutations at the -35 hexamer (-35 to -30) and a novel conserved upstream element (UPE: -45 to -36, +1 referring to the transcription start point) were constructed. Expression of the mutants was examined in vivo and in vitro by analyses using a beta-galactosidase assay, primer extension, and a DNA-mobility shift assay with RNA polymerases. Results indicated that the -35 hexamer and its proximal UPE act as effective cis-elements for the light-responsive and/or basal transcription, respectively. A model of the 5'-upstream region with cis- and possible trans-acting factors is presented for the psbA regulatory system.