The Rieske [2Fe)2S] protein (ISP) is an essential subunit of cyto-chromebc1 complexes in mitochondrial and bacterial respiratory chains. Based on the presence of two consecutive arginines, it was argued that the ISP ofParacoccus denitrificans, a Gram-negative soil bacterium, is inserted into the cytoplasmic membrane via the twin-arginine translocation (Tat) pathway.