Abstract
Cyanobacterial PII protein is known to regulate the nitrate transporter, nitrate reductase and N-acetylglutamate kinase, but its function has not been fully understood. We previously showed that ammonium assimilation is activated by high light in a PII-dependent manner in Synechocystis sp. PCC6803. However, it was unclear whether PII upregulates the activities of the ammonium assimilation enzymes or whether it increases the supply of 2-oxoglutarate (2-OG), the acceptor of the newly-fixed nitrogen. We have now found that the PII-deficient mutant shows enhanced expression of the NtcA-regulated nitrogen assimilation genes under high light. Since NtcA is activated by 2-OG, it was deduced that 2-OG accumulates in the PII-deficient mutant under high light. We therefore infer that PII activates the ammonium assimilation enzyme(s) under the high light conditions.
