Uridine and uracil transport in Escherichia coli and transport-deficient mutants

Mutants of Escherichia coli K-12 which are defective in components of transport systems for uracil and uridine were isolated and utilized to characterize the transport mechanism of uracil and uridine. Mutant U-, isolated from a culture of the parent strain, is resistant to 5-fluorouracil and is deficient in the uracil transport system. Mutant UR-, isolated from a culture of the parent strain, is resistant to a low concentration of showdomycin and lacks the capacity to transport intact uridine. Mutant U-UR- isolated from a culture of mutant U-, is resistant to a low concentration of showdomycin and is defective in both uracil and intact uridine transport processes. Mutant UR-R- was isolated from a culture of mutant UR-, and is resistant to a high concentration of showdomycin. This mutant is defective for transport of intact uridine and addition lacks the transport system for the ribose moiety of uridine. Characteristics of uracil and uridine transport in parent and mutant cells demonstrate the existence of specific transport processes for uracil, intact uridine and the uracil and ribose moieties of uridine. Mutants U- and UR-, which are defective for uracil transport, lack uracil phosphoribosyltransferase activity and retain a small but significant capacity to transport uracil. The data support the conclusion that uracil is transported by two mechanisms, the major one of which requires uracil phosphoribosyltransferase activity, while the other process involves the transport of uracil as such. The characteristics of uridine transport in parent and mutant strains show that, in addition to transport as the intact nucleoside, uridine is rapidly cleaved to the uracil and ribose moieties. The latter is transported into the cell by a process which, in contrast to transport of intact uridine, does not require an energy source. The uracil moiety is released into the medium and is transported by the uracil transport system. Whole cells of the parent and mutant strains differ in their ability to cleave uridine even though cell-free extracts of all the strains have similar uridine phosphorylase activity. The data implicate a uridine cleavage enzyme in a group transport of the ribose moiety of uridine, a process which is nonfunctional in mutants which lack the capacity to transport the ribose moiety of uridine. A common transport component for this process and the transport of intact uridine is indicated by similarities in the inhibitory effects of heterologous nucleosides on these processes.