Abstract
The bacterial genomes of Thermotoga species show evidence of significant interdomain horizontal gene transfer from the Archaea. Members of this genus acquired many genes from the Thermococcales, which grow at higher temperatures than Thermotoga species. In order to study the functional history of an interdomain horizontally acquired gene we used ancestral sequence reconstruction to examine the thermal characteristics of reconstructed ancestral proteins of the Thermotoga lineage and its archaeal donors. Several ancestral sequence reconstruction methods were used to determine the possible sequences of the ancestral Thermotoga and Archaea myo-inositol-3-phosphate synthase (MIPS). These sequences were predicted to be more thermostable than the extant proteins using an established sequence composition method. We verified these computational predictions by measuring the activities and thermostabilities of purified proteins from the Thermotoga and the Thermococcales species, and eight ancestral reconstructed proteins. We found that the ancestral proteins from both the archaeal donor and the Thermotoga most recent common ancestor recipient were more thermostable than their descendants. We show that there is a correlation between the thermostability of MIPS protein and the optimal growth temperature (OGT) of its host, which suggests that the OGT of the ancestors of these species of Archaea and the Thermotoga grew at higher OGTs than their descendants.
Highlights
From the publication of the first genome sequence of a member of the bacterial order Thermotogales, that of Thermotoga (Tt.) maritima; the importance of gene sharing with the Archaea was apparent for this lineage [1]
In order to examine the characteristics of ancestral proteins inherited by interdomain horizontal gene transfer (HGT), the Thermotoga genomes were screened for a gene of archaeal origin whose protein product met the following criteria
We chose to examine the thermal characteristics of Thermotoga myo-inositol-3-phosphate synthase (MIPS; EC 5.5.1.4) [17], a protein that shows a clear phylogenetic origin in the Archaea
Summary
From the publication of the first genome sequence of a member of the bacterial order Thermotogales, that of Thermotoga (Tt.) maritima; the importance of gene sharing with the Archaea was apparent for this lineage [1]. Subsequent examinations of the genomes from the other Thermotogales showed that these organisms have extensively shared genes with the Archaea through horizontal gene transfer (HGT) [2]. Many of these archaeal genes were derived from the Thermococcales, mostly represented in modern species as hyperthermophiles with optimal growth temperatures (OGTs) above those of Thermotogales species. The genes inherited by the Thermotogales likely encoded proteins catalytically active at temperatures higher than that at which modern Thermotogales grow. The ancestral Thermotogales that inherited these genes likely grew at temperatures higher than modern species and so was suited to use them [2]. To examine the evolution of proteins acquired from the Archaea, we chose to reconstruct an ancestral protein, a myo-inositol-3-phosphate synthase (MIPS), shown to have been acquired by Thermotoga species from the Archaea (Nesbo et al 2001)
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