Isolation and characterization of a sulfur-regulated gene encoding a periplasmically localized protein with sequence similarity to rhodanese.

During sulfur-limited growth, the cyanobacterium Synechococcus sp. strain PCC 7942 loses most of its photosynthetic pigments and develops an increased capacity to acquire sulfate. Sulfur deprivation also triggers the synthesis of several soluble polypeptides. We have isolated a prominent polypeptide of 33 kDa that accumulates specifically under sulfur-limiting conditions. This polypeptide was localized to the periplasmic space. The gene for this protein (designated rhdA) was isolated and discovered to lie within a region of the Synechococcus sp. strain PCC 7942 genome that encodes components of the sulfate permease system. The mRNA for the 33-kDa protein accumulates to high levels within an hour after the cells are deprived of sulfur and drops rapidly when sulfur is added back to the cultures. The amino acid sequence of the protein has similarity to bovine liver rhodanese, an enzyme that transfers the thiol group of thiosulfate to a thiophilic acceptor molecule, and a rhodaneselike protein of Saccharopolyspora erythraea. A strain in which rhdA was interrupted by a drug resistance marker exhibited marginally lower levels of rhodanese activity but was still capable of efficiently utilizing a variety of inorganic sulfur sources. The possible role of this protein in the transport of specific sulfur compounds is discussed.