초록
<P>Adaptive evolution was employed to generate sodium (Na<SUP>+</SUP>)-tolerant mutants of <I>Escherichia coli</I> MG1655. Four mutants with elevated sodium tolerance, designated ALS1184, ALS1185, ALS1186, and ALS1187, were independently isolated after 73 days of serial transfer in medium containing progressively greater Na<SUP>+</SUP> concentrations. The isolates also showed increased tolerance of K<SUP>+</SUP>, although this cation was not used for selective pressure. None of the adapted mutants showed increased tolerance to the nonionic osmolyte sucrose. Several physiological parameters of <I>E. coli</I> MG1655 and ALS1187, the isolate with the greatest Na<SUP>+</SUP> tolerance, were calculated and compared using glucose-limited chemostats. Genome sequencing showed that the ALS1187 isolate contained mutations in five genes, <I>emrR</I>, <I>hfq</I>, <I>kil</I>, <I>rpsG</I>, and <I>sspA</I>, all of which could potentially affect the ability of <I>E. coli</I> to tolerate Na<SUP>+</SUP>. Two of these genes, <I>hfq</I> and <I>sspA</I>, are known to be involved in global regulatory processes that help cells endure a variety of cellular stresses. Pyruvate formate lyase knockouts were constructed in strains MG1655 and ALS1187 to determine whether increased Na<SUP>+</SUP> tolerance afforded increased anaerobic generation of lactate. In fed-batch fermentations, <I>E. coli</I> ALS1187 <I>pflB</I> generated 76.2 g/liter lactate compared to MG1655 <I>pflB</I>, which generated only 56.3 g/liter lactate.</P>