Regulation of methylammonium/ammonium transport in the unicellular cyanobacterium Synechococcus R-2 (PCC 7942)

The unicellular cyanobacterium Synechococcus R-2 (PCC 7942) took up ¹⁴CH₃NH⁺₃ at least 10 times faster after growth with NO^?₃ than with NH⁺₄ as nitrogen source. When NH⁺₄-grown cells were transferred to medium containing NO^?₃, the ability to accumulate ¹⁴CH₃NH⁺₃ required approximately 6 h for maximal expression. Chloramphenicol (50 μg · ml^?1) completely prevented the rise in transport ability. Transport by nitrate-grown cells incubated in 2 mM NH₄Cl or CH₃NH₃Cl, with or without NO^?₃, continued at high rates for 2–3 h, but fell to zero by about 5 h incubation in light. Methylammonium, or the γ-methyl glutamic amide derived from it, thus may also serve as a signal for repression; however, externally added glutamine stimulated rather than repressed ¹⁴CH₃NH⁺₃ transport. Changes in ¹⁴CH₃NH⁺₃ uptake rates brought about by NH⁺₄ were confined to these slow effects, and to competition for entry into the cell.