Fig. 1.

Dual function of SgrS under glucose phosphate stress. (Middle) Glucose is transported and phosphorylated into the cell by IICB^Glc. When G6P accumulates abnormally, the synthesis of SgrS is induced, depending on SgrR. (Right) SgrS forms a ribonucleoprotein complex associating with Hfq and RNase E to act on the ptsG mRNA encoding IICB^Glc through base-pairing, resulting in translational inhibition and RNase E-dependent rapid degradation of the message (17). The physiological role of SgrS-mediated riboregulation of ptsG mRNA is to limit accumulation of toxic sugar phosphates by stopping new synthesis of IICB^Glc. (Left) As shown by Wadler and Vanderpool (3) in this issue of PNAS, SgrS acts also as an mRNA template for a small functional protein SgrT to prevent glucose uptake, presumably by inhibiting the transport activity of IICB^Glc. The SgrT function should be useful to rapidly prevent glucose uptake by inhibiting preexisting IICB^Glc, whereas the riboregulation through base-pairing would be important for adaptation to prolonged stress. It is not known whether one SgrS molecule is involved in both base-pairing and SgrT production or different SgrS molecules exert the two functions separately.