A sensory transducer homologous to the mammalian peripheral-type benzodiazepine receptor regulates photosynthetic membrane complex formation in Rhodobacter sphaeroides 2.4.1.

The Rhodobacter sphaeroides 2.4.1 tryptophan-rich sensory protein gene, tspO (formerly crtK, ORF160) encodes a 17-kDa protein which has an unusually high content of aromatic amino acids in general and of L-tryptophan in particular. The TspO protein was localized to the outer membrane of aerobically grown R. sphaeroides 2.4.1 by use of a polyclonal antibody against the purified protein. This protein is present in severalfold higher levels in photosynthetic as opposed to aerobic grown cells. Although tspO lies within the crt gene cluster, null mutations have an intact carotenoid biosynthetic pathway. In the TSPO1 mutant there was an increased production of carotenoids and bacteriochlorophyll relative to the wild type, particularly when cells were grown aerobically or semiaerobically. When present in trans the tspO gene restored "normal" pigment production to TSPO1. The effect of the tspO gene on pigment production was shown to take place at the level of gene expression. Because the tspO gene product of R. sphaeroides 2.4.1 shows significant sequence homology and similarity to the peripheral-type benzodoazepine receptor from mammalian sources, TspO-specific antibodies when probed against liver and kidney mitochondrial protein showed strong cross-reactivity. The role of TspO in R. sphaeroides 2.4.1 and its relation to photosynthesis gene expression are discussed.