Na+ gradient-coupled porters of Escherichia coli share a common subunit

D. Zilberstein, I. J. Ophir, E. Padan, S. Schuldiner

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Abstract

The phs mutation on the chromosome of Escherichia coli pleiotropically affects all transport systems which are dependent on the Na+ gradient. The Na+/H+ anti-porter in the mutant DZ3 was shown to be impaired in its sodium extrusion capacity. It is now demonstrated that the glutamate/Na+ and melibiose/Na+ symporters are also defective in the mutant. The K[m] for Na+ of the glutamate system increased 10-fold and there was a similar increase in the K[m] for glutamate. Furthermore, the efflux of glutamate down its respective chemical gradient was also impaired in DZ3, strongly suggesting that the lesion is in the translocation system itself. Other membrane-related functions such as growth on glycerol and succinate, generation of membrane potential, alkaline inside ΔpH, transport of substrates dependent on the electrochemical gradient of H+, namely lactose and proline, and even the N+-independent transport of melibiose and glutamate were all normal in DZ3. The pleiotropic effect of the phs locus is explained by a model previously suggested for Bacillus alcalophilus, assuming a dimeric structure of the Na+-utilizing transport systems. One subunit encoded by the phs locus is common to all these systems and is responsible for the Na+ translocation. The other subunit translocating the specific substrates is encoded by different genes and is individual to each system. This model is strongly supported by the demonstration that the two genes glt and phs in E. coli determine transport activity of glutamate. Furthermore, the products of these genes, when introduced in the trans position in a merodiploid, complement each other to yield normal transport activity.

Original languageEnglish
Pages (from-to)3692-3696
Number of pages5
JournalJournal of Biological Chemistry
Volume257
Issue number7
StatePublished - 1982

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