Bacterial origin for the isoprenoid biosynthesis enzyme HMG-CoA reductase of the archaeal orders thermoplasmatales and archaeoglobales

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reducta se or HMGR) fulfills an essential role in archaea, as it is required for th e synthesis of isoprenoid ethers, the main component of archaeal cell membr anes. There are two clearly homologous but structurally different classes o f the enzyme, one found mainly in eukaryotes and archaea (class 1), and the other found in bacteria (class 2). This feature facilitated the identifica tion of several cases of interdomain lateral gene transfer (LGT), in partic ular, the bacterial origin for the HMGR gene from the archaeon Archaeoglobu s fulgidus. In order to investigate if this LGT event was recent and limite d in its scope of had a broad and long-term impact on the recipient and its related lineages, the HMGR gene was amplified and sequenced From a variety of archaea. The survey covered close relatives of A. fulgidus, the only ar chaeon known prior to this study to possess a bacterial-like HMGR; represen tatives of each main euryarchaeal group were also inspected. All culturable members of the archaeal group Archaeoglobales were found to display an HMG R very similar to the enzyme of the bacterium Pseudomonas mevalonii. Surpri singly, two species of the genus Thermoplasma also harbor an HMGR of bacter ial origin highly similar to the enzymes found in the Archaeoglobales. Phyl ogenetic analyses of the HMGR gene and comparisons to reference phylogenies from other genes confirm a common bacterial origin for the HMGRs of Thermo plasmatales and Archaeoglobales. The most likely explanation of these resul ts includes an initial bacteria-to-archaea transfer, followed by a another event between archaea. Their presence in two divergent archaeal lineages su ggests an important adaptive role for these laterally transferred genes.