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

In this paper a gene (psfR) is reported that regulates psbAl activity in Synechococcus elongatus, a unicellular photoautotrophic cyanobacterium that carries out oxygenic (plant-type) photosynthesis and exhibits global circadian regulation of gene expression. In S. elongatus, a family of three psbA genes encodes the D1 protein of the photosystem II reaction centre. Overexpression of psfR results in increased expression of psbAl, but does not affect the circadian timing of psbAl expression. psfR overexpression affected some, but not all of the genes routinely surveyed for circadian expression. PsfR acts (directly or indirectly) on the psbAl basal promoter region, psfR knockout mutants exhibit wild-type psbAl expression, suggesting that other factors can regulate psbAl expression 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. l also contains a GGDEF domain. The presence of these domains and the absence of a detectable conserved DNA-binding domain suggest that PsfR may regulate psbAl expression via protein-protein interactions or GGDEF activity (the production of cyclic dinucleotides) rather than direct interaction with the psbAl promoter.