We have isolated and characterized a nuclear mutant, 222E, in Chlamydomonas reinhardtii, which is defective in photosystem II (PSII). Polypeptide P5, the product of psbB, is not produced in this mutant, leading to a destabilization of other PSII components. The mutant specifically fails to accumulate psbB transcripts and displays an altered transcription pattern downstream of psbB. Pulse-labelling experiments suggest that mRNA stability and/or processing are affected by the alteration of a nuclear gene product in this mutant. We show that the C. reinhardtii psbB gene is co-transcribed with a small open reading frame that is highly conserved in location and amino acid sequence in land plants. The 5′ and 3′ termini of the psbB transcript have been mapped to 35 bases upstream of the initiation codon and approximately 600 bases downstream of the stop codon. The 3′ flanking region contains two potential stem-loops, of which the larger (with an estimated free energy of −46 kcal) is near the 3′ terminus of the transcript.