Chloroplast photogene 32 codes for the 32-kilodalton triazine herbicide-binding protein at the B site of electron transport in photosystem II of the photosynthetic apparatus-its product is the B protein and the gene is accordingly designated ps2B here. The cyanobacteria Anacystis nidulans R2, Fremyella diplosiphon, and Nostoc sp. MAC each contain several copies of ps2B. The sequence of one copy of ps2B from Fremyella, ps2B-1, has been determined. The longest open reading frame would code for a protein of 360 amino acids. Although the deduced amino acid sequence of ps2B-1 is highly homologous overall to that of the corresponding spinach protein [Zurawski, G., Bohnert, H. J., Whitfeld, P. R. & Bottomley, W. (1982) Proc. Natl. Acad. Sci. USA 79, 7699-7703] and, excluding neutral substitutions, the homology is 95% for an internal segment of 309 amino acids, there are a number of nonneutral amino acid substitutions. Most of the differences in net charge and polarity occur in the first 20 amino acids at the amino terminus and in the amino acid composition at the carboxyl terminus. The nucleotide sequences are 76% homologous overall. Conserved sequences resembling prokaryotic "-10" and "-35" regions are found at remarkably similar positions in the spinach and F. diplosiphon sequences although the surrounding sequences show only occasional homologies.