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