Tryptophan-rich sensory protein (TspO) is an integral membrane protein that acts as a negative regulator of the expression of specific photosynthesis genes in response to oxygen/light [1]. It is involved in the efflux of porphyrin intermediates from the cell. This reduces the activity of coproporphyrinogen III oxidase, which is thought to lead to the accumulation of a putative repressor molecule that inhibits the expression of specific photosynthesis genes. Several conserved aromatic residues are necessary for TspO function: they are thought to be involved in binding porphyrin intermediates [3]. In [2], the rat mitochondrial peripheral benzodiazepine receptor (MBR) was shown to not only retain its structure within a bacterial outer membrane, but also to be able to functionally substitute for TspO in TspO- mutants, and to act in a similar manner to TspO in its in situ location: the outer mitochondrial membrane. The biological significance of MBR remains unclear, however. It is thought to be involved in a variety of cellular functions, including cholesterol transport in steroidogenic tissues.

1: A sensory transducer homologous to the mammalian peripheral-type benzodiazepine receptor regulates photosynthetic membrane complex formation in Rhodobacter sphaeroides 2.4.1. Yeliseev AA, Kaplan S; J Biol Chem 1995;270:21167-21175. PMID:7673149

2: A mammalian mitochondrial drug receptor functions as a bacterial oxygen sensor. Yeliseev AA, Krueger KE, Kaplan S; Proc Natl Acad Sci U S A 1997;94:5101-5106. PMID:9144197

3: TspO of rhodobacter sphaeroides. A structural and functional model for the mammalian peripheral benzodiazepine receptor. Yeliseev AA, Kaplan S; J Biol Chem 2000;275:5657-5667. PMID:10681549