Primary structure of the Synechococcus PCC 7942 PAPS reductase gene

The structural gene encoding a thioredoxin-dependent 5'-phosphoadenylyl sulphate (PAPS) reductase (EC 1.8.4.-) from cyanobacterium Synechococcus PCC 7942 ('Anacystis nidulans') was detected by heterologous hybridization with the cysH gene from Escherichia coli K12. The cyanobacterial gene (further called par gene) comprised 696 nt which are 57.8% homologous to the enterobacterial gene. The putative open reading frame encoded a polypeptide consisting of 232 amino acid residues (deduced molecular weight 26,635) which showed significant homologies to the polypeptide from E. coli (50.8%) and to the polypeptide from Saccharomyces cerevisiae (30.3%). A single cysteine located at the C-terminus of the polypeptide of E. coli (Cys239) was conserved in Synechococcus. Conservation of this cysteinyl residue seems indispensable for catalysis. Complementation of a cysH-deficient mutant of E. coli by the cyanobacterial gene indicated that the cloned DNA is the structural gene of the PAPS reductase.