The PSI-H subunit of photosystem I has two isoforms of differing molecular mass in Nicotiana sylvestris [Obokata et al. (1993) Plant Physiol. 102: 1259], and is encoded by a nuclear gene, psaH. We identified three structurally distinct psaH genes in the nuclear genome of N. sylvestris, designated psaHa, psaHb, and psaHc, and all three genes are expressed in young leaves. Each gene has two introns: one between sequences encoding a transit peptide and the N-terminal acidic domain, and one between the N-terminal domain and a central hydrophobic domain. The deduced amino acid sequences are identical in the mature proteins and differ only in the transit peptides. Since PSI-H is present in two isoforms in N. sylvestris, the psaH products may be subjected to post-translational modifications.