Engineering yeast endosymbionts as a step toward the evolution of mitochondria.

Abstract

It has been hypothesized that mitochondria evolved from a bacterial ancestor that initially became established in an archaeal host cell as an endosymbiont. Here we model this first stage of mitochondrial evolution by engineering endosymbiosis between <i>Escherichia coli</i> and <i>Saccharomyces cerevisiae</i> An ADP/ATP translocase-expressing <i>E. coli</i> provided ATP to a respiration-deficient <i>cox2</i> yeast mutant and enabled growth of a yeast-<i>E. coli</i> chimera on a nonfermentable carbon source. In a reciprocal fashion, yeast provided thiamin to an endosymbiotic <i>E. coli</i> thiamin auxotroph. Expression of several SNARE-like proteins in <i>E. coli</i> was also required, likely to block lysosomal degradation of intracellular bacteria. This chimeric system was stable for more than 40 doublings, and GFP-expressing <i>E. coli</i> endosymbionts could be observed in the yeast by fluorescence microscopy and X-ray tomography. This readily manipulated system should allow experimental delineation of host-endosymbiont adaptations that occurred during evolution of the current, highly reduced mitochondrial genome.