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

Comparison of the rate of accumulation of Rb 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 Rb uptake to charge uptake was approx. 1.0, suggesting that the transport system operates as a uniporter. The addition of NHto a cell suspension led to an increase in membrane ionic current but failed to inhibit the accumulation of Rb during illumination. The presence of K and NHinhibited 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 NHcan be transported by the K carrier and in the context of an alternative hypothesis that NHincreases the affinity of the K transport system for its natural substrate and for Rb. © 1992 Springer-Verlag.