In this paper a gene (psfR) is reported that regulates psbAI 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 psbAI, but does not affect the circadian timing of psbAI expression. psfR overexpression affected some, but not all of the genes routinely surveyed for circadian expression. PsfR acts (directly or indirectly) on the psbAI basal promoter region. psfR knockout mutants exhibit wild-type psbAI expression, suggesting that other factors can regulate psbAI 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. 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 regulate psbAI expression via protein-protein interactions or GGDEF activity (the production of cyclic dinucleotides) rather than direct interaction with the psbAI promoter.