Summary: A gene cluster containing homologues of the genes and was found in the genome of the sulphur-oxidizing purple bacterium The nucleotide sequence indicated four open reading frames encoding homologues of 3′-phosphoadenylylsulphate (PAPS) reductase (CysH), sulphite reductase flavoprotein (CysJ) and haem protein (Cysl) subunits, and a transcriptional regulator (CysB). Genes are separated by a short -active intergenic region from which is transcribed divergently. encodes a polypeptide of 35·9 kDa consisting of 323 amino acid residues with 40% identity to the CysB regulator from enterobacteria. encodes a protein with 239 amino acid residues and a calculated mass of 27·7 kDa; encodes a protein with 522 amino acid residues and a mass of 57·8 kDa; and encodes a protein with 559 amino acid residues and a mass of 62·3 kDa. The gene products have been expressed and used for complementation of mutants from Biochemical analysis. The gene product CysH is a thioredoxin-dependent PAPS reductase (EC It was repressed under photoautotrophic growth using hydrogen sulphide as electron donor and derepressed under conditions of sulphate deficiency. Products of the genes were identified as the two subunits of NADPH-sulphite reductase (EC encoded the flavoprotein, with 39% identity to the protein from and encoded the haem protein, with 53% identity. A clone was used to complement the corresponding mutant from and to express enzymically active methylviologen-sulphite reductase.


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