Multiple Molecular Forms of Uridine Diphosphate Glucose Pyrophosphorylase from Salmonella typhimurium

A wild type strain of Salmonella typhimurium produced three forms (II, IIIa, and IIIb) of uridine diphosphate glucose pyrophosphorylase (EC 2.7.7.9), the molecular weights of which were around 40,000, 40,000, and 80,000, as determined by centrifugation in a sucrose density gradient. A deletion mutant his-519, lacking gene galF, produced a single species (IV) of the enzyme with a molecular weight around 80,000. Enzyme IIIa was converted in vitro into Enzyme II; this conversion required a factor that was present also in the cells lacking the galF gene. Enzyme IIIa was also converted into a dimer, IIIb, and this conversion was accelerated by small amounts of NaCl. Enzymes II, IIIb, and IV were stable and were not further converted into any other form at neutral pH. It is suggested that the structural gene for uridine diphosphate glucose pyrophosphorylase, galU, codes for a polypeptide which forms a tightly associated dimer (Enzyme IV) in the absence of products specified by the gene galF. In the wild type cells, however, the products coded for by the galF gene modify the galU-specified polypeptide so that it can now exist as a monomer (Enzyme IIIa), which in turn can be further converted into Enzyme II or IIIb.