Exobiology and Evolutionary Biology

Genome-based Investigations into the Nature of the Common Ancestor of the Thermotogales (2)

PI: Kenneth Noll

This project will provide experimental evidence that can be used to address a fundamental question about the conditions under which life evolved on the early Earth. That question, was the ancestor of all life a thermophile, has had a controversial history. A central component in support of the thermophilic origin hypothesis is the fact that many of the most thermophilic extant microbes are apparently the closest descendants of the common ancestor of all life. One of those important groups, the bacterial lineage Thermotogales, has recently been shown to have non-thermophilic members. This raises the possibility that the extant Thermotogales species are actually descendants of nonthermophilic ancestors. In the study outlined here, we propose to examine this question using newly available genome sequence data, sophisticated bioinformatics tools, synthesis of hypothetical ancestral genes, and examinations of the functions of putative ancestral proteins.

The proposed research will take advantage of the rapidly growing number of genome sequences of Thermotogales species to address questions about the nature of the ancestors within this lineage. In the near future, twelve genome sequences of species of the Thermotogales will be available. Using phylogenetic genome and gene reconstruction methods, we will determine which genes found in these genomes likely were present in those ancestors. We will examine those genes for evidence of thermophilic features of their encoded proteins. A few hypothetical proteins that would be encoded in these ancestral genomes will be chosen for actual synthesis in the lab. Those proteins will be examined for their thermal stability and functions at elevated temperatures. If they exhibit features associated with modern thermostable proteins, this will provide evidence about the nature of these hypothetical ancestors. In a related set of studies, we will use sequences of ligand binding proteins of growth substrate transporters to examine questions about growth substrates of these ancestors and explore how the ligand specificities of these proteins evolved.