20The cell wall polymers wall teichoic acid (WTA) and lipoteichoic acid (LTA) are often 21 modified with glycosyl and D-alanine residues. Recent studies have shown that a three-22 component glycosylation system is used for the modification of LTA in several Gram-positive 23 bacteria including Bacillus subtilis and Listeria monocytogenes. In the L. monocytogenes 1/2a 24 strain 10403S, the cytoplasmic glycosyltransferase GtlA is thought to use UDP-galactose to 25 produce the C55-P-galactose lipid intermediate, which is transported across the membrane by 26 an unknown flippase. Next, the galactose residue is transferred onto the LTA backbone on the 27 outside of the cell by the glycosyltransferase GtlB. Here we show that GtcA is necessary for 28 the glycosylation of LTA in L. monocytogenes 10403S and B. subtilis 168 and we hypothesize 29 that these proteins act as C55-P-sugar flippases. With this we revealed that GtcA is involved in 30 the glycosylation of both teichoic acid polymers in L. monocytogenes 10403S, namely WTA 31 with N-acetylglucosamine and LTA with galactose residues. These findings indicate that the 32 L. monocytogenes GtcA protein can act on different C55-P-sugar intermediates. Further 33 characterization of GtcA in L. monocytogenes led to the identification of residues essential for 34 its overall function as well as residues, which predominately impact WTA or LTA 35 glycosylation. 36 37 GRAPHICAL ABSTRACT 38 39 40 41 most S. aureus strains, Listeria monocytogenes strains, and B. subtilis strain W23 produce 76 WTA with a ribitolphosphate (RboP) backbone (Brown et al., 2010, Weidenmaier and Peschel, 77
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