4-HYDROXYBENZOYL-COA REDUCTASE (DEHYDROXYLATING) FROM THE DENITRIFYING BACTERIUM THAUERA-AROMATICA - PROSTHETIC GROUPS, ELECTRON-DONOR, ANDGENES OF A MEMBER OF THE MOLYBDENUM-FLAVIN-IRON-SULFUR PROTEINS

4-Hydroxybenzoyl-CoA reductase catalyzes an important reaction in the anaerobic metabolism of phenolic compounds, i.e, the reductive removal of an aromatic hydroxyl group. The prosthetic groups and the natural electron donor of the enzyme were investigated and the genes were clon ed and sequenced. The enzyme is a molybdenum-flavin-iron-sulfur protei n of subunit composition of alpha(2) beta(2) gamma(2).It contains appr oximately 1.3 flavin nucleotide, probably FAD, 1.9 Mo, 15 Fe, and 12.5 acid-labile sulfur. Sequence interpretation suggests that the native enzyme contains two [4Fe-4S] and four [2Fe-2S] clusters. A 9.8-kDa fer redoxin with two [4Fe-4S] clusters functions as the natural electron d onor. The genes coding for the three subunits, hcrABC, show high simil arities to other molybdenum-flavin-iron-sulfur proteins of the xanthin e oxidase family, notably to the three putative 4-hydroxybenzoyl-CoA r eductase genes in Rhodopseudomonas palustris. In addition, there are c lose similarities to three open reading frames (orf) in E. coli. A maj or difference is the presence of an additional domain in the beta-subu nit (HcrB, 35 kDa) probably carrying an additional iron-sulfur cluster , The 82-kDa alpha-subunit (HcrA) contains a Mo-cofactor-binding site. The 17-kDa gamma-subunit (HcrC) harbors two [2Fe-2S] clusters, Upstre am of the hcrCAB region, an ORF was found coding for a regulatory prot ein of the MarR family, Downstream of the hcrCAB region lies an ORF pr esumably coding for a hydrophobic permease.