Sugar Transport in Neurospora crassa

Cells of Neurospora crassa grown in media containing low levels of glucose produce a transport system which catalyzes the accumulation of l-sorbose against a considerable concentration gradient. This system is physiologically a glucose active transport system which operates maximally at extremely low glucose concentrations (Km for glucose, approximately 10 µm). The active glucose transport system is distinct from a previously described facilitated diffusion glucose transport system which functions at high glucose concentrations (Km for glucose, approximately 8 mm). Only the facilitated diffusion system is present in cells grown on high levels of glucose, whereas the active transport system appears when cells are grown on low levels of glucose. Repression of the high affinity active transport system at high glucose concentrations and derepression of this system at low glucose concentrations provides the organism with an efficient mechanism for maintaining adequate intracellular glucose concentrations regardless of extreme fluctuations in the glucose content of the external environment.