The ascorbate transporter of Escherichia coli

The sgaTBA genes of Escherichia coli encode a putative 12-transmembrane alpha-helical segment (12 TMS) transporter, an enzyme IIB-like protein and an enzyme IIA-like protein of the phosphotransferase system (PTS), respectively. We show that all three proteins as well as the energy-coupling PTS proteins, enzyme I and HPr, are required for the anaerobic utilization and uptake Of L-ascorbate in vivo and its phosphoenolpyruvate-dependent phosphorylation in vitro. The transporter exhibits an apparent K. for L-ascorbate of 9 muM and is highly specific. The sgaTBA genes are regulated at the transcriptional level by the yjfQ gene product, as well as by Crp and Fnr. The yjfR gene product is essential for L-ascorbate utilization and probably encodes a cytoplasmic L-ascorbate 6-phosphate lactonase. We conclude that SgaT represents a novel prototypical enzyme IIC that functions with SgaA and SgaB to allow phosphoryl transfer from HPr(his-P) to L-ascorbate via the phosphoryl transfer pathway: PEP --> enzyme I-P --> HPr-P --> IIA-P-SgaA --> IIB-P-SgaB -->(IIC SgaT) L-ascorbate-6-P.