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

Abstract

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 NHinf4sup+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 NHinf4sup+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 NHinf4sup+can be transported by the K+ carrier and in the context of an alternative hypothesis that NHinf4sup+increases the affinity of the K+ transport system for its natural substrate and for Rb+.