A 991 bp DNA fragment, consisting of a 225 amino acid reading frame homologous to outer membrane protein coding ompA gene, was cloned from a purple sulfur bacterium Allochromatium vinosum. The homology analysis revealed up to 51% similarity to other bacterial species. The absence of branching within diazotrophs or other taxonomically related groups shows the structural importance of the protein regardless of the metabolism and evolution of the species. (C) 2006 Elsevier GmbH. All rights reserved.