The psbP gene encoding the precursor of 23-kDa polypeptide of the oxygen-evolving complex of photosystem II has been isolated from Arabidopsis thaliana genomic library and sequenced. The gene harbors three introns and encodes a mature polypeptide of 186 amino acid residues and a transit peptide of 77 amino acid residues. The deduced molecular mass of the mature polypeptide is 23.5-kDa and it contains 22.6% charged amino acid residues which may contribute to the hydrophilic nature of the protein. The transcript encoded by psbP gene of Arabidopsis is approximately 1.3-kb long. In wild-type Arabidopsis seedlings, its expression is organ-specific and is regulated by endogenous developmental cues, light and sucrose. In a constitutively photomorphogenic mutant of Arabidopsis, designated as pho1, the psbP gene is partly derepressed in young, dark-grown seedlings, resulting in a slightly higher level of the transcript. Additionally, the pho1 mutant shows slow accumulation of psbP transcript upon illumination of young, dark-grown seedlings. However, the derepression is not markedly displayed on dark-adaptation of pho1 plants grown in continuous light. These studies, therefore, define the activity of at least one cellular effector involved in regulation of psbP expression.