A gene cluster containing homologues of the genes cysB, cysJI and cysH was found in the genome of the sulphur-oxidizing purple bacterium Thiocapsa roseopersicina. The nucleotide sequence indicated four open reading frames encoding homologues of 3'-phosphoadenylylsulphate (PAPS) reductase (CysH), sulphite reductase flavoprotein (CysJ) and haem protein (CysI) subunits, and a transcriptional regulator (CysB). Genes cysJIH are separated by a short cis-active intergenic region from cysB which is transcribed divergently. cysB encodes a polypeptide of 35.9 kDa consisting of 323 amino acid residues with 40% identity to the CysB regulator from enterobacteria. cysH encodes a protein with 239 amino acid residues and a calculated mass of 27.7 kDa; cysJ encodes a protein with 522 amino acid residues and a mass of 57.8 kDa; and cysI encodes a protein with 559 amino acid residues and a mass of 62.3 kDa. The cysJIH gene products have been expressed and used for complementation of cys mutants from Escherichia coli Biochemical analysis. The gene product CysH is a thioredoxin-dependent PAPS reductase (EC 1.8.99.4). It was repressed under photoautotrophic growth using hydrogen sulphide as electron donor and derepressed under conditions of sulphate deficiency. Products of the cysJI genes were identified as the two subunits of NADPH-sulphite reductase (EC 1.8.1.2). cysJ encoded the flavoprotein, with > or = 39% identity to the protein from E. coli, and cysI encoded the haem protein, with > or = 53% identity. A cysI clone was used to complement the corresponding mutant from E. coli and to express enzymically active methylviologen-sulphite reductase.