Signal-transduction protein PII from Synechococcus elongatus PCC 7942 senses low adenylate energy charge in vitro

P(II) proteins belong to a family of highly conserved signal-transduction proteins that occurs widely in bacteria, archaea and plants. They respond to the central metabolites ATP, ADP and 2-OG (2-oxoglutarate), and control enzymes, transcription factors and transport proteins involved in nitrogen metabolism. In the present study, we examined the effect of ADP on in vitro P(II)-signalling properties for the cyanobacterium Synechococcus elongatus, a model for oxygenic phototrophic organisms. Different ADP/ATP ratios strongly affected the properties of P(II) signalling. Increasing ADP antagonized the binding of 2-OG and directly affected the interactions of P(II) with its target proteins. The resulting P(II)-signalling properties indicate that, in mixtures of ADP and ATP, P(II) trimers are occupied by mixtures of adenylate nucleotides. Binding and kinetic activation of NAGK (N-acetyl-L-glutamate kinase), the controlling enzyme of arginine biosynthesis, by P(II) was weakened by ADP, but relief from arginine inhibition remained unaffected. On the other hand, ADP enhanced the binding of P(II) to PipX, a co-activator of the transcription factor NtcA and, furthermore, antagonized the inhibitory effect of 2-OG on P(II)-PipX interaction. These results indicate that S. elongatus P(II) directly senses the adenylate energy charge, resulting in target-dependent differential modification of the P(II)-signalling properties.