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The Ascorbate Transporter of Escherichia coli

Division of Biological Sciences, University of California at San Diego, La Jolla, California 92093-0116,¹ Department of Microbiology and Immunology, Faculty of Pharmacy, Ain Shams University, Abbassia, Cairo, Egypt²

Received 21 October 2002/ Accepted 16 January 2003

The sgaTBA genes of Escherichia coli encode a putative 12-transmembrane -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_m for L-ascorbate of 9 µM 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- IIB- L-ascorbate-6-P.

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