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.