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 NH ^sup+_inf4 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 ^sup+_inf4 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 ^sup+_inf4 can be transported by the K⁺ carrier and in the context of an alternative hypothesis that NH ^sup+_inf4 increases the affinity of the K⁺ transport system for its natural substrate and for Rb⁺.