The sulfate reducing bacteria Desulfovibrio vulgaris and the methanogenic archaea Methanosarcina barkeri can grow syntrophically on lactate. In this study, a set of three closely located genes, DVU2103, DVU2104, and DVU2108 of D. vulgaris, was found to be up-regulated 2- to 4-fold following the lifestyle shift from syntroph to sulfate reducer, moreover, none of the genes in this gene set were differentially regulated when comparing gene expression from various D. vulgaris pure culture experiments. Although exact function of this gene set is unknown, the results suggest that it may play roles related to the lifestyle change of D. vulgaris from syntroph to sulfate reducer. This hypothesis is further supported by phylogenomic analyses showing that homologies of this gene set were only narrowly present in several groups of bacteria, most of which are restricted to a syntrophic lifestyle, such as Pelobacter carbinolicus, Syntrophobacter fumaroxidans, Syntrophomonas wolfei, and Syntrophus aciditrophicus. Phylogenetic analysis showed that all three individual genes in the gene set tended to be clustered with their homologies from archaeal genera, and they were rooted on archaeal species in the phylogenetic trees, suggesting that they were horizontally transferred from archaeal methanogens. In addition, no significant bias in codon and amino acid usages was detected between these genes and the rest of the D. vulgaris genome, suggesting the gene transfer may have occurred early in the evolutionary history so that sufficient time has elapsed to allow an adaptation to the codon and amino acid usages of D. vulgaris. This report provides novel insights into the origin and evolution of bacterial genes linked to the lifestyle change of D. vulgaris from a syntrophic to a sulfate-reducing lifestyle. (c) 2006 Elsevier Inc. All rights reserved.