CHARACTERIZATION OF THE GENE CYSH AND OF ITS PRODUCT PHOSPHO-ADENYLYLSULPHATE REDUCTASE FROM ESCHERICHIA-COLI

The nucleotide sequence1 of the gene cysH from Escherichia coli K12 was determined. The open reading frame was 735 nucleotides in length; it was flanked by a repetitive palindromic sequence centred 36 nucleotides upstream of cysH and a terminator-like structure located 20 nucleotides downstream. CysH encoded a polypeptide of M(r) 27927 consisting of 244 amino acids. The gene product was isolated as a homodimer exhibiting phospo-adenylylsulphate reductase (PAPS reductase) activity. The active enzyme was devoid of electron transferring cofactors and contained only one cysteine per subunit. Reduction of the enzyme by dithiols resulted in a shift of the apparent molecular weight from 44000 to 62000 without formation of an enzyme-thioredoxin complex.