(278a) Regulation of Xylose and Arabinose Metabolism by Escherichia Coli

Microorganisms can be used to produce a variety of chemicals such as drugs, enzymes, and fuels from different sugars. Traditionally, these processes have involved a single feedstock, most often glucose. More recently, significant effort has been devoted towards developing processes that directly use plant-based material as a feedstock. One challenge presented by the use of plant-based material is it is comprised of multiple sugars, each with a unique biochemistry. In particular, can these microorganisms process mixtures of sugars and, if not, is it possible to engineer them so that they can efficiently complete this task?

In this work, we investigated how Escherichia coli (E. coli), a common industrial microorganism, uptakes and then metabolizes mixtures of glucose, arabinose, and xylose, the main constituents in plant-based material. Our specific focus was on understanding how the metabolic pathways for arabinose and xylose are transcriptionally regulated. In particular, E. coli will only activate a specific pathway if the target sugar is present. Furthermore, different pathways can interfere with one another in such a way that the utilization of one sugar prevents that of another.

Using both genetic and analytical approaches, we found that E. coli will not simultaneously metabolize mixtures of glucose, arabinose, and xylose. Instead, E. coli will sequentially utilize them based on their energy content, starting with glucose and finishing with xylose. From an industrial standpoint, this hierarchy means that the fermentation of these sugar mixtures may be inefficient and necessitate complex processing schemes.

While the mechanism for glucose catabolite repression is well known, little is known about the hierarchy between arabinose and xylose. In order to understand the mechanism of how E. coli sequentially utilizes these sugars, we systematically removed, over-expressed, and mutated key metabolic and transporter genes in the arabinose pathway. The results from these experiments allowed us to conclude that repression is due to AraC, the arabinose transcriptional activator, and not inducer exclusion.

Collectively, this work has uncovered a complex cellular control system employed by E. coli in order to selectively utilize individual sugars. By identifying the mechanism for this control, we have identified a specific target for subsequent metabolic engineering. This discovery may facilitate the design of E. coli strains capable of simultaneously and efficiently processing mixtures of glucose, arabinose, and xylose. Such strains will likely have application in large-scale production of lignocellulosic biofuels.