Archives of Microbiology

, Volume 157, Issue 2, pp 125–130 | Cite as

Evidence that the low-affinity K+ transport system of Rhodobacter capsulatus is a uniporter. The effects of ammonium on the transporter

  • Paul Golby
  • Nicholas P. J. Cotton
  • Mark Carver
  • J. Baz Jackson
Original Papers


Comparison of the rate of accumulation of 86Rb+ by intact cells of Rhodobacter capsulatus during short periods of illumination with the Rb+-dependent membrane ionic current measured by electrochromism supports the view that both activities reflect the operation of a low-affinity K+ transport system. In experiments performed under similar conditions the ratio of 86Rb+ uptake to charge uptake was approx. 1.0, suggesting that the transport system operates as a uniporter. The addition of NH inf4 sup+ to a cell suspension led to an increase in membrane ionic current but failed to inhibit the accumulation of 86Rb+ during illumination. The presence of K+ and NH inf4 sup+ inhibited the increase in cellular ATP levels at the onset of illumination. This effect was prevented by Cs+. The results are considered within the context of the hypothesis (Golby et al. Eur J Biochem 187: 589–597) that NH inf4 sup+ can be transported by the K+ carrier and in the context of an alternative hypothesis that NH inf4 sup+ increases the affinity of the K+ transport system for its natural substrate and for Rb+.

Key words

Potassium ion transport Ammonium transport Protonmotive force Membrane potential Membrane current Uniport Photosynthetic bacteria (Rhodobacter capsulatus



pH difference across the cytoplasmic membrane


electrical potential difference across the cytoplasmic membrane


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Copyright information

© Springer-Verlag 1992

Authors and Affiliations

  • Paul Golby
    • 1
  • Nicholas P. J. Cotton
    • 1
  • Mark Carver
    • 2
  • J. Baz Jackson
    • 1
  1. 1.School of BiochemistryUniversity of BirminghamBirminghamUK
  2. 2.ICI Biological ProductsBillinghamUK

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