Regulation of purine utilization in bacteria. VII. Involvement of membrane-associated nucleoside phosphorylase in the uptake and the base-mediated loss of the ribose moiety of nucleosides by Salmonella typhimurium membrane vesicles

Abstract

Although uridine and adenosine are converted by membrane-associated nuf cleoside phosphorylases to ribose-1-phosphate (ribose-1-P) and the corresponding bases (uracil and adenine), only ribose-1-P is accumulated within Salmonella typhimurium LT2 membrane vesicles. In accordance with these observations, no uptake is observed when the vesicles are incubated with the bases or nucleosides labeled in their base moieties. The vesicles lack a transport system for ribose-1-P, since excess ribose-1-P does not inhibit the uptake of the ribose moiety of uridine. In addition, there is no exchange with preaccumulated ribose-1-P. Thus, uridine, rather than ribose-1-P, must serve as the initially transported substrate. The uptake of the ribose portion of uridine is coupled to electron transport, and the levels to which ribose-1-P are accumulated may be reduced by adding various bases to the reaction mixtures. The bases appear to inhibit the uridine phosphorylase reaction and/or cause an efflux of ribose-1-P from the vesicles. This loss of ribose-1-P reflects the accumulation of nucleosides in the external medium after being synthesized within the membranes. Synthesis of the nucleosides from intravesicular ribose-1-P and exogenous bases proceeds even though the bases are not accumulated by the vesicles. Furthermore, ribose-1-P cannot significantly inhibit uridine phosphorylase activity unless the membranes are disrupted. These observations indicate that the membrane-associated nucleoside phosphorylases may have a transmembranal orientation with their base and ribose-1-P binding sites on opposite sides of the membranes. Such an asymmetric arrangement of these enzymes may facilitate the uptake of the ribosyl moiety ofnucleosides by a group translocation mechanism. Thus, nucleosides may be cleaved during the membrane transport process, with the resultant bases delivered to the external environment while ribose-1-P is shunted to the intravesicular space.

Role of membrane-bound 5'-nucleotidase in nucleotide uptake by the moderate halophile Vibrio costicola

“…Although numerous studies of nucleoside uptake by bacteria have appeared, the views as to the mechanisms of uptake are conflicting. One group of investigators reached a conclusion that nucleosides are transported without prior catabolism (8,10,11,19), whereas other workers postulated that adenosine is cleaved by phosphorolysis before uptake (7,17,25). The results presented in this paper suggest that in V. costicola adenosine is taken up without prior catabolism.…”

Bacterial phosphoenolpyruvate: sugar phosphotransferase systems: structural, functional, and evolutionary interrelationships.

“…Another enzyme which apparently catalyzes group translocation binds adenosine on the outer membrane surface and releases adenine into the external medium, but translocates the ribose moiety across the membrane (23). The intracellular product released by this enzyme is ribose-1-P where the phosphoryl moiety is derived from inorganic phosphate.…”

Periplasmic enzymes in Bdellovibrio bacteriovorus and Bdellovibrio stolpii

“…Operationally, true periplasmic enzymes lie freely in this region, are not membrane associated, and are easily released by spheroplasting or osmotic shock (2). However, osmotic shock also releases enzymes such as aminopeptidase N (18), purine phosphorylase (24), and deoxyriboaldolase (16) that are not truly periplasmic but are membrane associated and not easily released by spheroplasting. Penicillin treatment of bdellovibrio cells converts them into motile spheroplasts that are essentially devoid of the peptidoglycan and osmotically stable in the absence of sucrose in the suspending buffer (28).…”