Evolution and function of Archaea-derived oligopeptide transporter genes in the Thermotogales

A large number of open reading frames (ORFs) in T. maritima appear to have archaeal genes as closest homologs and may have been acquired by horizontal gene transfer (HGT). Among those ORFs are several encoding putative oligopeptide (Opp) ATP binding cassette (ABC) transporters. ABC transporters are the preferred transport machinery in T. maritima. Data resulting from extensive phylogenetic analyses are presented here suggesting that six of the eleven Opp ABC transporters are homologs of archaeal transporters. These transporter genes group among archaeal ABC transporter genes along with those from some thermophilic bacteria. These results showed that these transporters were acquired through at least two HGT events from the Thermococcales and from relatives of Archaeoglobus or Aeropyrum. Walking PCR was performed to survey other members of the Thermotogales for the presence of these operons. Those results suggested that the transfer events might have occurred before speciation of the Thermotogales since many of the transporter operons are present in all fifteen members tested. ^ To better understand the impact of HGT on the evolution of the physiology of these organisms, the substrates of the binding proteins encoded by these Opp transporters were determined. These Opp transporter operons are adjacent to genes encoding extracellular sugar hydrolytic enzymes, so they may facilitate oligosaccharide transport. Changes in intrinsic fluorescence of these binding proteins were measured in response to substrate availability and oligosaccharides were found to bind with high affinities. No proteins bound monosaccharides. In general, the substrates of the binding proteins were the likely products of the extracellular hydrolases encoded nearby. ^ Horizontally transferred genes must be beneficial to the recipient cells to be passed on to their offspring. T. maritima's Opp transporters appear to provide it with the ability to utilize oligosaccharides and so may have broadened its spectrum of growth substrates. These new genes must have increased the fitness of the ancestor of T. maritima that acquired them and so enhanced its survival in the environment. ^