ORGANIZATION AND EXPRESSION OF THE RHODOBACTER-SPHAEROIDES CYCFG OPERON

The Rhodobacter sphaeroides cycFG operon has been cloned, sequenced, a nd mapped to approximately coordinate 2500 of chromosome I. The cycF g ene encodes cytochrome c(554), a member of the class II family of solu ble cytochrome c proteins. The cycF open reading frame includes a 20-a mino acid extension at its N terminus which has not been detected in c ytochrome c(554). Antiserum against cytochrome c(554) shows that this protein is localized to the periplasm of wild-type cells, which sugges ts that this N-terminal extension functions as a signal peptide, The p redicted cycG gene product is a diheme cytochrome c with a subunit mol ecular mass of similar to 32 kDa. While a cytochrome with the properti es predicted for CycG has not been reported for R. sphaeroides, we hav e tentatively identified this protein as a heme-staining polypeptide t hat is associated with membranes. CycG could have an overall structure similar to that of several other electron carriers, since the similar ity between the predicted amino acid sequence of CycG and other multih eme cytochrome c proteins extends throughout the polypeptide. The cycF G transcript is similar to 1,500 nucleotides long and has a single 5' end 26 nucleotides upstream of the start of cycF translation. Expressi on of cycFG is regulated at the level of mRNA accumulation, since appr oximately fivefold-higher levels of both cycF-specific transcript and cytochrome c(554) protein are detected in cell extracts from aerobic c ultures in comparison with those from anaerobically grown cells. Altho ugh cytochrome c(554) was detected under all growth conditions tested, the highest levels of this protein were found when cells generate ene rgy via aerobic respiration.