K. Pattaragulwanit et al., MOLECULAR-GENETIC EVIDENCE FOR EXTRACYTOPLASMIC LOCALIZATION OF SULFUR GLOBULES IN CHROMATIUM-VINOSUM, Archives of microbiology, 169(5), 1998, pp. 434-444
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.