MOLECULAR-GENETIC EVIDENCE FOR EXTRACYTOPLASMIC LOCALIZATION OF SULFUR GLOBULES IN CHROMATIUM-VINOSUM

Purple sulfur bacteria store sulfur as intracellular globules enclosed by a protein envelope. We cloned the genes sgpA, sgpB, and sgpC, whic h encode the three different proteins that constitute the sulfur globu le envelope of Chromatium vinosum D (DSMZ 180(T)). Southern hybridizat ion analyses and nucleotide sequencing showed that these three genes a re not clustered in the same operon. All three genes are preceded by s equences resembling sigma(70)-dependent promoters, and hairpin structu res typical for rho-independent terminators are found immediately down stream of the translational stop codons of sgpA, sgpB, and sgpC. Inser tional inactivation of sgpA in Chi: vinosum showed that the presence o f only one of the homologous proteins SgpA and SgpB suffices for forma tion of intact sulfur globules. All three sgp genes encode translation products which - when compared to the isolated proteins - carry amino -terminal extensions. These extensions meet all requirements for typic al signal peptides indicating an extracytoplasmic localization of the sulfur globule proteins. A fusion of the phoA gene to the sequence enc oding the proposed signal peptide of sgpA led to high specific alkalin e phosphatase activities in Escherichia coli, further supporting the e nvisaged targeting process. Together with electron microscopic evidenc e these results provide strong indication for an extracytoplasmic loca lization of the sulfur globules in Chi: vinosum and probably in other Chromatiaceae. Extracytoplasmic formation of stored sulfur could contr ibute to the transmembranous Delta p that drives ATP synthesis and rev erse electron flow in Chr. vinosum.