Evidence for a threshold of error-induced extinction.

<p>A) Entry into error-induced extinction. Mutated <i>pol3</i> alleles were introduced into haploid <i>MSH6</i> and <i>msh6Δ</i> yeast by plasmid shuffling (<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1002282#pgen.1002282.s001" target="_blank">Figure S1</a>), and mutation rates were measured by fluctuation assays and calculated using the maximum likelihood method. Each bar represents the spontaneous mutation rate, expressed as canavanine-resistant (Can<sup>r</sup>) mutants per cell division, conferred by a specific <i>POL3</i> allele in <i>MSH6</i> or <i>msh6Δ</i> cells. Mutation rate values (x 10<sup>−7</sup>) are indicated on each column. Error bars show 95% confidence intervals. WT, wild-type <i>POL3</i>; black, <i>MSH6</i>; gray, <i>msh6Δ</i>; <b>X</b>, no growth. B) Synthetic lethality of strong <i>pol3</i> mutator alleles with <i>msh6Δ</i>. Serial dilutions of haploid yeast containing <i>POL3–URA3</i> and <i>pol3–LEU2</i> plasmids were plated on SC FOA medium to select for cells that spontaneously lost <i>POL3–URA3</i>. Similar numbers of cells (∼10<sup>5</sup>, 10<sup>4</sup> and 10<sup>3</sup>) were plated for each set of alleles in the <i>MSH6</i> and <i>msh6Δ</i> strains. Failed growth of <i>msh6Δ</i> cells carrying <i>pol3-D407A</i> or <i>pol3-01</i> indicates synthetic lethality (right two panels). <i>pol3-F406A</i> and <i>pol3-Y516F</i> also failed to support colony formation in <i>msh6Δ</i> cells (not shown). Note the small size of <i>pol3-D463A msh6Δ</i> colonies.</p>

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