PsfR, a factor that stimulates psbAI expression in the cyanobacterium Synechococcus elongatus PCC 7942

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Abstract
In this paper a gene (psfR) is reported that regulatespsbAIactivity inSynechococcus elongatus, a unicellular photoautotrophic cyanobacterium that carries out oxygenic (plant-type) photosynthesis and exhibits global circadian regulation of gene expression. InS. elongatus, a family of threepsbAgenes encodes the D1 protein of the photosystem II reaction centre. Overexpression ofpsfRresults in increased expression ofpsbAI, but does not affect the circadian timing ofpsbAIexpression.psfRoverexpression affected some, but not all of the genes routinely surveyed for circadian expression. PsfR acts (directly or indirectly) on thepsbAIbasal promoter region.psfRknockout mutants exhibit wild-typepsbAIexpression, suggesting that other factors can regulatepsbAIexpression in the absence of functional PsfR. PsfR contains two receiver-like domains (found in bacterial two-component signal transduction systems), one of which lacks the conserved aspartyl residue required for phosphoryl transfer. PsfR also contains a GGDEF domain. The presence of these domains and the absence of a detectable conserved DNA-binding domain suggest that PsfR may regulatepsbAIexpression via protein–protein interactions or GGDEF activity (the production of cyclic dinucleotides) rather than direct interaction with thepsbAIpromoter.