Phosphate transport in Neurospora. Derepression of a high-affinity transport system during phosphorus starvation

In addition to the constitutive, low-affinity phosphate-transport system described previously, Neurospora possesses a second, high-affinity system which is derepressed during phosphorus starvation. At pH 5.8, System II has a of about 3μM and a J_max of 5.2 mmol/l cell water per min.System II reaches maximal activity after about 2 h of growth in phosphorus-free minimal medium. Its formation is blocked by cycloheximide and, once made, it appears to turn over rapidly. Addition of cycloheximide to fully derepressed cultures results in the decay of System II with a of 14 min, very similar to the turnover rate previously reported (Wiley, W.R. and Matchett, W.H. (1968) J. Bacteriol. 95, 959–966) for tryptophan transport in Neurospora. Thus, these transport systems appear to be regulated by a balance between synthesis and breakdoown, as affected by intracellular pools of substrate or related compounds.