May play a role in the transmembrane transport of tetrapyrroles and similar compounds, and thereby contribute to the regulation of tetrapyrrole biosynthesis (PubMed:10409680). Binds tetrapyrroles and promotes the photooxidative degradation of protoporphyrin IX (PubMed:23651039). Binds protoporphyrin IX, hemin, and coproporphyrin III, but does not bind delta-aminolevulinic acid (PubMed:23952237, PubMed:20541505, PubMed:25635101). Can bind bilirubin, curcumin, gossypol, retinoic acid, cholesterol and the benzodiazepine receptor agonist PK-11195 (in vitro) (PubMed:23952237, PubMed:25635101). Plays a role in the response to low oxygen levels and in the regulation of the biosynthesis of photosynthetic pigments (PubMed:7673149, PubMed:10409680, PubMed:10681549). Homodimer. Outer membrane proteins generally mediate transport processes by forming beta-barrel structures, and unlike TspO, their transmembrane domains do not contain any helices. Detected in the cell inner membrane when expressed in E.coli (PubMed:20541505). This suggest that TspO is in the inner membrane, even if experiments shown in PubMed:7673149 suggest location in the outer membrane. Up-regulated during photosynthetic growth, when compared to aerobic growth in the dark (at protein level). No effect on chemoheterotrophic growth, and no defects in the production of photosynthetic pigments. When oxygen levels are reduced during chemoheterotrophic growth, mutant cells respond more rapidly than wild-type and display faster accumulation of carotenoids and bacteriochlorophylls. Belongs to the TspO/BZRP family.