Role of the phosphoenolpyruvate-dependent fructose phosphotransferase system in the utilization of mannose by Escherichia coli

Abstract

Mutants of Escherichia coli devoid of the membrane-spanning proteins PtsG and PtsMP, which are components of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) and which normally effect the transport into the cells of glucose and mannose, do not grow upon or take up either sugar. Pseudorevertants are described that take up, and grow upon, mannose at rates strongly dependent on the mannose concentration in the medium (apparent Km > 5 mM); such mutants do not grow upon glucose but are derepressed for the components of the fructose operon. Evidence is presented that mannose is now taken up via the fructose-PTS to form mannose 6-phosphate, which is further utilized for growth via fructose 6-phosphate and fructose 1,6-bisphosphate.

Memoirs of a Biochemical Hod Carrier

“…Being fascinated by the flexibility by which E. coli can overcome all manner of metabolic handicaps by evolving alternative routes for fructose utilization, I chose to make this general area the focus of my work, hoping thereby to gain an insight into the mechanisms by which this unicellular organism senses what is "out there" and adjusts its intracellular machinery accordingly. So far, this has shown us that the principal fructose transporter FruA can also admit mannose (82) and that the principal glucose transporter can mutate to effect the facilitated diffusion of fructose (33), this latter process being also the first step in a novel route for fructose utilization that does not involve the PT system (15). I am currently studying further mutants in which fructose supports growth despite the absence of the components of the PT system and of a functioning glucose carrier, profoundly grateful for the opportunity still to carry (and occasionally drop) bricks as small contributions to the grand edifice of biochemistry.…”

“…It had been given additional impetus by the recognition of a "pecking order" in the sequence in which different hexoses are taken up by E. coli. For example, as little as micromolar amounts of glucose will powerfully inhibit the continued uptake of fructose, even though the cells are fully induced for fructose utilization and that ketose is present in great excess (68); however, mutants can be isolated in which this preference for glucose has been abolished (69). The site of one such mutation lay not in a gene for glucose utilization but was co-transducible with the gene(s) specifying the fructose operon, the major route of fructose uptake.…”