Structure of the Bacillus anthracis dTDP-l-rhamnose-biosynthetic enzyme glucose-1-phosphate thymidylyltransferase (RfbA)

l-Rhamnose is a ubiquitous bacterial cell-wall component. The biosynthetic pathway for its precursor dTDP-l-rhamnose is not present in humans, which makes the enzymes of the pathway potential drug targets. In this study, the three-dimensional structure of the first protein of this pathway, glucose-1-phosphate thymidylyltransferase (RfbA), from Bacillus anthracis was determined. In other organisms this enzyme is referred to as RmlA. RfbA was co-crystallized with the products of the enzymatic reaction, dTDP-α-d-glucose and pyrophosphate, and its structure was determined at 2.3Å resolution. This is the first reported thymidylyltransferase structure from a Gram-positive bacterium. RfbA shares overall structural characteristics with known RmlA homologs. However, RfbA exhibits a shorter sequence at its C-terminus, which results in the absence of three α-helices involved in allosteric site formation. Consequently, RfbA was observed to exhibit a quaternary structure that is unique among currently reported glucose-1-phosphate thymidylyltransferase bacterial homologs. These structural analyses suggest that RfbA may not be allosterically regulated in some organisms and is structurally distinct from other RmlA homologs.The crystal structure of glucose-1-phosphate thymidylyltransferase (RfbA) from B. anthracis was determined at 2.3Å resolution in complex with the reaction products dTDP-α-d-glucose and pyrophosphate. RfbA is the first enzyme of the dTDP-l-rhamnose pathway and does not exhibit the structural features that form the allosteric site in its closest RmlA homologs.