Discrimination between Rb+ and K+ by Escherichia coli

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Escherichia coli requires potassium (K+) for essential cellular functions.
• Rubidium (Rb+) is a K+ analog but its efficacy in fulfilling K+ requirements is not fully understood.
• E. coli possesses multiple K+ transport systems with varying affinities and roles.

Purpose of the Study:

• To investigate the extent to which Rb+ can substitute for K+ in supporting E. coli growth.
• To characterize the discrimination of Rb+ by the different K+ transport systems in E. coli.
• To evaluate the accuracy of using 86Rb+ as a tracer for K+ influx studies.

Main Methods:

• Culturing E. coli in media supplemented with Rb+ instead of K+ to assess growth.
• Measuring molar growth yields and growth rates under different ionic conditions.
• Utilizing 86Rb+ as a tracer to quantify K+ influx through individual transport systems.
• Calculating the discrimination factors of each transport system against the Rb+ tracer.

Main Results:

• Molar growth yield on Rb+ was only 5% of that on K+.
• Growth rates were significantly lower in Rb+-supplemented media compared to K+.
• The high-affinity Kdp system showed extreme discrimination against 86Rb+ (at least 1000-fold).
• Moderate affinity systems (TrkA, TrkD) exhibited 10-fold and 25-fold discrimination, respectively.
• The low-rate TrkF system preferred 86Rb+, overestimating K+ influx by 40%.

Conclusions:

• Rb+ is a poor substitute for K+ in E. coli, supporting only minimal growth.
• The four characterized K+ transport systems display significant differences in their ability to discriminate against Rb+.
• 86Rb+ is an unreliable tracer for quantifying K+ influx, particularly for the Kdp, TrkA, and TrkD systems, due to substantial discrimination.