Polyprenylphosphate-arabinose (in which the polyprenyl unit is found both as decaprenyl and octahydroheptaprenyl) is a donor of mycobacterial cell wall arabinosyl residues. Because of this important role, its biosynthetic pathway, and that of the related lipid, polyprenylphosphate-D-ribose, was investigated. Further experiments showed that the mature polyprenylphosphate-ribose is formed from phosphoribose pyrophosphate via a two-step pathway involving a transferase to form polyprenylphosphate-5-phosphoribose and then a phosphatase to form the final polyprenylphosphateribose. Polyprenylphosphate-arabinose is formed by a similar pathway with an additional step being the epimerization at C-2 of the ribosyl residue. This epimerization occurs at either the level of phosphoribose pyrophosphate or at the level of polyprenylphosphate-5-phosphoribose.The mycobacterial cell wall core consists of a highly impermeable layer of the unique 70 -90 carbon mycolic acids covalently attached to an inner peptidoglycan layer by way of the connecting polysaccharide, arabinogalactan. Arabinogalactan consists of three regions: the linker region (1), which is connected to the peptidoglycan, a galactan directly attached to the linker (2), and an arabinan (2), which is directly attached to the galactan. The mycolic acids are attached at the non-reducing end (3) of the arabinan. Since the arabinan is fundamental to the structural integrity of the cell wall, its biosynthesis has recently been studied (4 -9) with an ultimate aim of developing new tuberculosis drugs targeted at one or more of the arabinose biosynthetic enzymes.A major breakthrough (4) in these studies was the isolation of polyprenylphosphate-arabinose in the form of decaprenylphosphate-arabinose and octahydroheptaprenylphosphatearabinose. Subsequently, it was demonstrated that radioactive decaprenylphosphate-arabinose made chemically (5), or a mixture of radioactive decaprenylphosphate-arabinose and octahydroheptaprenylphosphate-arabinose, isolated from cultures of Mycobacterium smegmatis, 1 could function as arabinosyl donors in the presence of M. smegmatis enzymes to form polymeric arabinan. Hence, determining the pathway of the biosynthesis of polyprenylphosphate-arabinose itself becomes important for the ultimate goal of developing new drugs against mycobacteria.In a related area, mycobacteria have been shown (9) to synthesize polyprenylphosphate-ribose. This compound has been characterized as decaprenylphosphate-ribose (9), although it is likely to exist as octahydroheptaprenylphosphate-ribose as well. The function of this glycolipid is not yet clear. It does not appear to be a biosynthetic precursor of polyprenylphosphatearabinose because radioactive polyprenylphosphate-ribose is not converted by mycobacterial enzymes to polyprenylphosphate-arabinose (9). Polyprenylphosphate-ribose may function as a ribosyl donor (9) since mycobacteria have been shown to ribosylate certain antibiotics (10).Polyprenylphosphate sugars are generally synthesized by the transfer of a glycosy...