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.