Transport of purines and deoxyadenosine in Escherichia coli

Abstract

The characteristics of adenine, guanine, hypoxanthine, xanthine, and uracil uptake in Escherichia coli B show that each base is transported by a specific system. The data support the concept that the transport of guanine, hypoxanthine, xanthine, and uracil function without direct involvement of the respective purine or pyrimidine phosphoribosyltransferase enzymes. Uracil phosphoribosyltransferase is not demonstrable in E. coli B, and large differences are observed in the inhibitory effects of heterologous purines on the uptake of guanine, hypoxanthine, and xanthine as compared to the corresponding inhibitory effects reported for the soluble purine phosphoribosyltransferase enzymes of E. coli B. Additional evidence is provided by the low Km values determined for the transport of adenine, guanine, hypoxanthine, and xanthine relative to the corresponding Km values for the phosphoribosyltransferase enzymes. Data are presented indicating that adenine may be transported without participation of adenine phosphoribosyltransferase. The stimulatory effect of glucose, the inhibitory effect of KCN, and the high intracellular to extracellular concentration gradients of the bases produced in the presence of glucose provide evidence that the transport processes are energy-dependent. The Km values for transport of the purines and uracil range from 10(-7) M to 5 X 10(-7) M. Characteristics of adenine and uracil uptake are similar in E. coli B, E. coli K-12, and a showdomycin-resistant mutant of E. coli B. Adenosine and deoxyadenosine are transported in E. coli B by independent transport systems. Adenine or hypoxanthine does not share the adenosine or deoxyadenosine transport systems as evidence by the mutual lack of competition of free bases and nucleosides on transport. The transport systems for deoxyadenosine and adenosine are defective in the mutant.