The gene encoding a membrane protein, SspA, induced under salt stress conditions was cloned and sequenced from a photosynthetic bacterium, Rhodobacter sphaeroides f. sp. denitrificans IL106. A single open reading frame consisting of 972 base pairs that encoded a polypeptide composed of a signal peptide of 24 amino acids and a mature protein of 300 amino acids (Mr 33,386) was found. A database search failed to detect any highly homologous sequences, indicating that SspA is a novel protein. The protein was present in the outer membrane as a transmembrane protein and was specifically induced by salt stress, but not by heat shock.