Complex media are routinely used to cultivate diverse bacteria. However, this complexity can obscure the factors that govern cell growth. While studying protein acetylation in buffered tryptone broth supplemented with glucose (TB7-glucose), we observed that Escherichia coli did not fully consume glucose prior to stationary phase. However, when we supplemented this medium with magnesium, the glucose was completely consumed during exponential growth, with concomitant increases in cell number and biomass but reduced cell size. Similar results were observed with other sugars and other peptide-based media, including lysogeny broth. Magnesium also limited cell growth for Vibrio fischeri and Bacillus subtilis in TB7-glucose. Finally, magnesium supplementation reduced protein acetylation. Based on these results, we conclude that growth in peptide-based media is magnesium limited. We further conclude that magnesium supplementation can be used to tune protein acetylation without genetic manipulation. These results have the potential to reduce potentially deleterious acetylated isoforms of recombinant proteins without negatively affecting cell growth.IMPORTANCE Bacteria are often grown in complex media. These media are thought to provide the nutrients necessary to grow bacteria to high cell densities. In this work, we found that peptide-based media containing a sugar are magnesium limited for bacterial growth. In particular, magnesium supplementation is necessary for the bacteria to use the sugar for cell growth. Interestingly, in the absence of magnesium supplementation, the bacteria still consume the sugar. However, rather than use it for cell growth, the bacteria instead use the sugar to acetylate lysines on proteins. As lysine acetylation may alter the activity of proteins, this work demonstrates how lysine acetylation can be tuned through magnesium supplementation. These findings may be useful for recombinant protein production, when acetylated isoforms are to be avoided. They also demonstrate how to increase bacterial growth in complex media.KEYWORDS acetylation, complex media, improved cell growth, magnesium, tryptone B acteria are routinely grown in complex media containing a rich assortment of nutrients. For example, lysogeny broth (LB) contains tryptone and yeast exact. Together, these components provide a rich mixture of nutrients that supports growth of numerous bacteria. However, the complexity of such media obscures factors governing cell behavior, especially those that limit cell growth.We routinely grow Escherichia coli aerobically in buffered tryptone broth (TB7) supplemented with 4 g/liter glucose (TB7-glucose) when studying protein acetylation.

Lysine acetylation is thought to provide a mechanism for regulating metabolism in diverse bacteria. Indeed, many studies have shown that the majority of enzymes involved in central metabolism are acetylated and that acetylation can alter enzyme activity. However, the details regarding this regulatory mechanism are still unclear, specifically with regard to the signals that induce lysine acetylation. To better understand this global regulatory mechanism, we profiled changes in lysine acetylation during growth of Escherichia coli on the hexose glucose or the pentose xylose at both high and low sugar concentrations using label-free mass spectrometry. The goal was to see whether lysine acetylation differed during growth on these two different sugars. No significant differences, however, were observed. Rather, the initial sugar concentration was the principal factor governing changes in lysine acetylation, with higher sugar concentrations causing more acetylation. These results suggest that acetylation does not target specific metabolic pathways but rather simply targets accessible lysines, which may or may not alter enzyme activity. They further suggest that lysine acetylation principally results from conditions that favor accumulation of acetyl phosphate, the principal acetate donor in E. coli. IMPORTANCE Bacteria alter their metabolism in response to nutrient availability, growth conditions, and environmental stresses using a number of different mechanisms. One is lysine acetylation, a posttranslational modification known to target many metabolic enzymes. However, little is known about this regulatory mode. We investigated the factors inducing changes in lysine acetylation by comparing growth on glucose and xylose. We found that the specific sugar used for growth did not alter the pattern of acetylation; rather, the amount of sugar did, with more sugar causing more acetylation. These results imply that lysine acetylation is a global regulatory mechanism that is responsive not to the specific carbon source per se but rather to the accumulation of downstream metabolites.A n abundant posttranslational modification in many bacteria is N -lysine acetylation (1,2). Multiple studies have shown that lysine acetylation predominantly targets the enzymes involved in central metabolism (2)(3)(4)(5). Because some of these lysines are catalytically active, their acetylation may regulate metabolism in bacteria. This hypothesis is supported by several in vitro studies showing that lysine acetylation indeed alters the activity of some enzymes involved in central metabolism (4, 6-8). However, it is still not clear what role lysine acetylation plays in regulating metabolism. One compelling model is that lysine acetylation provides a global mechanism by which cells regulate metabolism in response to their energy status. The response to energy status occurs Citation Schilling B,

Conclusion.We profiled protein acetylation in E. coli during growth on glucose and xylose 126 at both high and low sugar concentrations. We did not observe major differences amongst the 127 lysines acetylated during respective growth on these two sugars. Rather, the observed changes 128 7 in lysine acetylation were principally correlated with the initial sugar concentration, with higher 129 sugar concentrations causing more acetylation. These results indicate that acetylation is 130 agnostic to the metabolic route and simply targets accessible lysines, which may or may not 131 alter enzyme activity. They further support the hypothesis that lysine acetylation results from the 132 buildup of metabolic intermediates, principally acetyl phosphate, under conditions that favor 133 acetate production (Figure S1). 134 135 ACKNOWLEDGMENT 136

Escherichia coli produces acetate during aerobic growth on various carbon sources. After consuming the carbon substrate, E. coli can further grow on the acetate. This phenomenon is known as the acetate switch, where cells transition from producing acetate to consuming it. In this study, we investigated how pH governs the acetate switch. When E. coli was grown on a glucose-supplemented medium initially buffered to pH 7, the cells produced and then consumed the acetate. However, when the initial pH was dropped to 6, the cells still produced acetate but were only able to consume it when little (<10 mM) acetate was produced. When significant acetate was produced in acidic medium, which occurs when the growth medium contains magnesium, amino acids, and sugar, the cells were unable to consume the acetate. To determine the mechanism, we characterized a set of metabolic mutants and found that those defective in the tricarboxylic acid (TCA) cycle or glyoxylate shunt exhibited reduced rates of acetate consumption. We further found that the expression of the genes in these pathways was reduced during growth in acidic medium. The expression of the genes involved in the AckA-Pta pathway, which provides the principal route for both acetate production and consumption, was also inhibited in acidic medium but only after glucose was depleted, which correlates with the acetate consumption phase. On the basis of these results, we conclude that growth in acidic environments inhibits the expression of the acetate catabolism genes, which in turn prevents acetate consumption. IMPORTANCE Many microorganisms produce fermentation products during aerobic growth on sugars. One of the best-known examples is the production of acetate by Escherichia coli during aerobic growth on sugars. In E. coli, acetate production is reversible: once the cells consume the available sugar, they can consume the acetate previously produced during aerobic fermentation. We found that pH affects the reversibility of acetate production. When the cells produce significant acetate during growth in acidic environments, they are unable to consume it. Unconsumed acetate may accumulate in the cell and inhibit the expression of pathways required for acetate catabolism. These findings demonstrate how acetate alters cell metabolism; they also may be useful for the design of aerobic fermentation processes.

D-xylose is an abundant sugar found in plant biomass and can be used as a renewable feedstock for the microbial production of diverse biofuels and bioproducts. However, D-xylose metabolism is slow in many industrial microorganisms, at least as compared to glucose metabolism. Not surprisingly, a number of approaches have been developed for improving D-xylose metabolism in diverse microorganisms. In this work, we applied a previously developed evolution strategy based on mediain-oil emulsions for improving the growth yield of Escherichia coli NCM3722 on D-xylose. After 30 rounds of evolutions, we isolated multiple mutants with increased growth yield on D-xylose. In addition, we also observed similar increases in the growth rate. Three mutants were selected for whole-genome sequencing. Two mutants had an amber stop mutation in adenylate cyclase, which truncates nearly 60% of the enzyme. However, the ability of this mutant to grow on xylose indicated that truncated enzyme, lacking the C-terminal regulatory domain, is still active. The other mutant had a point mutation in the cyclic AMP receptor protein (CRP), near the high affinity binding site for cyclic AMP. Both mutations, when introduced into wild type E. coli, were able to increase the growth yields at levels similar to the isolated mutants. In addition to D-xylose, these mutant strains and their genetic mimics also exhibited higher growth rates and yields on glucose, lactose, and L-arabinose. These results suggest that the improved growth rates and yields are due to changes in the production and sensing of intracellular cyclic AMP concentrations and also suggest native concentrations are suboptimal with respect to the growth rate and yield under the growth conditions tested. Collectively, these results may prove useful for engineering strains of E. coli for high-density fermentations or protein production.