AGUILAR-ROCA, Nancy M.*; MOORE, Francisco B. -G.; SCHLUMBOHM, Jason P.; BENNETT, Albert F.; University of California, Irvine; University of Akron; University of California, Irvine; University of California, Irvine: Methionine biosynthesis limits adaptation to high temperature in Escherichia coli

In Escherichia coli, degradation of the first enzyme in the methionine biosynthesis pathway may be a critical limitation to growth at high temperatures. To determine if this pathway was subject to change by natural selection, we measured fitness and growth rate in 6 lines of E. coli that had evolved for 2000 generations at 41.5�C. In 3 of the 6 lines, fitness relative to their ancestor was significantly reduced with methionine supplementation compared to control. Thus, methionine partially restored the ability of the ancestor to compete by compensating for reduced methionine biosynthesis. In one line, methionine completely restored the fitness of the ancestor, suggesting that stabilizing the methionine pathway was a major adaptation to 41.5�C. In contrast, for those lines in which fitness was not significantly affected in methionine, adaptations to high temperature appear to be independent of methionine biosynthesis. If cultures were supplemented with alanine, fitness was unchanged or significantly increased, showing that changes in methionine are not a general amino acid effect. At 37�C, fitness changed significantly with addition of methionine in two lines, but in opposite directions. Lag time and maximal growth rate were measured at 41.5�C and at 37�C. Overall, lag time decreased and growth rate increased with methionine supplementation at 41.5�C, however no individual line changed significantly. Protecting methionine biosynthesis appears to be important during evolution at high temperatures, but the growth-phase in which it is critical isn�t clear. (Supported by NSF-IBN9905980 and NASA-632731 to AFB and NIH-Bioinformatics Training grant to NMA.)