Siro(haem)amide in Allochromatium vinosum and relevance of DsrL and DsrN, a homolog of cobyrinic acid a,c-diamide synthase, for sulphur oxidation

In the purple sulphur bacterium Allochromatium vinosum, the prosthetic group of dissimilatory sulphite reductase (DsrAB) was identified as siroamide, an amidated form of the classical sirohaem. The genes dsrAB are the first two of a large cluster of genes necessary for the oxidation of sulphur globules stored intracellularly during growth on sulphide and thiosulphate. DsrN is homologous to cobyrinic acid a,c diamide synthase and may therefore catalyze glutamine-dependent amidation of sirohaem. Indeed, an A. vinosum Delta dsrN in frame deletion mutant showed a significantly reduced sulphur oxidation rate that was fully restored upon complementation with dsrN in trans. Sulphite reductase was still present in the Delta dsrN mutant. DsrL is a homolog of the small subunits of bacterial glutamate synthases and was proposed to deliver glutamine for sirohaem amidation. However, recombinant DsrL does not exhibit glutamate synthase activity nor does the gene complement a glutamate synthase-deficient Escherichia coli strain. Deletion of dsrL showed that the encoded protein is absolutely essential for sulphur oxidation in A. vinosum.