Photosystem I (PSI) is a multisubunit pigment-protein complex in the thylakoid membranes of cyanobacteria and chloroplasts. BP26, a random photosynthesis-deficient mutant strain of the cyanobacterium Synechocystis 6803 has a severely reduced PSI content, whereas its photosystem II is present in normal amount. The BP26 mutant is complemented by a novel gene, btpA, that encodes a 30-kDa protein. In this strain, a missense mutation in the btpA gene, resulting in the replacement of a valine by a glycine residue, significantly affects the accumulation of the PSI reaction center proteins, PsaA and PsaB. Northern blot analysis revealed that the steady-state levels of the transcripts from the psaAB operon, encoding these proteins, remain unaltered in the mutant strain. Hence the BtpA protein regulates a post-transcriptional process during the life cycle of the PSI protein complex such as 1) translation of the psaAB mRNA, 2) assembly of the PsaA/PsaB polypeptides and their associated cofactors into a functional complex, or 3) degradation of the protein complex. Close relatives of the BtpA protein have been found in nonphotosynthetic organisms, viz. the archaebacterium Methanococcus jannaschii, the eubacterium Escherichia coli, and the nematode, Caenorhabditis elegans, suggesting that these proteins may regulate biogenesis of other protein complexes in these evolutionarily distant organisms.