THE PHO REGULON-DEPENDENT UGP UPTAKE SYSTEM FOR GLYCEROL-3-PHOSPHATE IN ESCHERICHIA-COLI IS TRANS INHIBITED BY P-I

sn-Glycerol-3-phosphate (G3P) or glyceryl phosphoryl phosphodiesters, the substrates of the phoB-dependent Ugp transport system, when transported exclusively through this system, can serve as a sole source of phosphate but not as a sole source of carbon (H. Schweizer, M. Argast, and W. Boos, J. Bacteriol. 150:1154-1163, 1982). In order to explain this phenomenon, we tested two possibilities: repression of the pho regulon by Ugp-mediated transport and feedback inhibition by internal G3P or its degradation product P-i. Using an ugp-lacZ fusion, we found that the expression of ugp does not decline upon exposure to G3P, in contrast to the repressing effect of transport of P-i via the Pst system. This indicated that the Ugp system becomes inhibited after the uptake and metabolism of G3P. Using P-32-labeled G3P, we observed that little P-i is released by cells taking up G3P via the Ugp system but large amounts of P-i are released when the cells are taking up G3P via the GlpT system. Using a glpD mutant that could not oxidize G3P but which could still phosphorylate exogenous glycerol to G3P after GlpF-mediated transport of glycerol, we could not find trans inhibition of Ugp-mediated uptake of exogenous C-14-G3P. However, when allowing uptake of P-i via Pst, we observed a time dependent inhibition of C-14-G3P taken up by the Ugp transport system. Inhibition was half maximal after 2 min and could be elicited by P-i concentrations below 0.5 mM. Cells had to be starved for P-i in order to observe this inhibition. We conclude that the activity of the Ugp transport system is controlled by the level of internal phosphate.