Pi entry into Escherichia coli cells through either of the two Pi-transport systems (Pit or Pst) prompts the influx of K+ and H+ in a ratio that depends on the external pH. The entry of Pi is absolutely dependent on the presence of K+, and the entry of K+ is equally dependent on the presence of Pi. Experiments with a number of mutants carrying any one functional Pi-transport system and one or more of the individual K+-transport systems indicate a permissive type of linkage of the two transports, in that there is no obvious preference by any of the Pi-transport systems for a particular K+-transport system for the concomitant entry of the two ions.
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