Characterization of the cys gene locus from Allochromatium vinosum indicates an unusual sulfate assimilation pathway

Homologues of the genes cysB, cysl, cysH, cysD, cysN, and selD were identif ied in the genome of the phototrophic purple sulfur bacterium Allochromatiu m vinosum (formerly Chromatium vinosum). On the basis of amino acid compari sons these genes encode a ferredoxin-dependent siroheme-sulfite reductase ( Cys1), a plant-type assimilatory APS reductase without thioredoxin dor-nain (CysH), the two different subunits of heterodimeric ATP sulfurylase (CysDN ), a transcriptional regulator (CysB) and a selenophosphate synthase (SelD) . cysIHDN appear to form an operon and are preceded by cysB which is transc ribed in the opposite direction. SelD is situated downstream of cysN and tr anscribed divergently to cysIHDN. The lack of a gene for APS kinase and pre sence of a gene for an assimilatory APS reductase implies that assimilatory sulfate reduction in A. vinosum proceeds along the pathway suggested for h igher plants without intermediary formation of PAPS. Two completely separat e pathways involving specialized enzymes are used for assimilatory sulfate reduction and dissimilatory sulfur oxidation in A. vinosum. The presence of cysB indicates that the genes for assimilatory sulfate reduction are expre ssed only in the absence of reduced sulfur compounds.