Okadaic acid mimics nitrogen-stimulated transcription of the NADH-glutamate synthase gene in rice cell cultures

Okadaic acid (OKA), a potent and specific inhibitor of protein serine/threonine phosphatases 1 and 2A, induced the accumulation of NADH-glutamate synthase (GOGAT) mRNA within 4 h in rice (Oryza sativa L.) cell cultures. In contrast to the transient accumulation of NADH-GOGAT mRNA by NH4+, OKA caused a continuous accumulation for at least 24 h. The induction of NADH-GOGAT mRNA by OKA was not inhibited in the presence of methionine sulfoximine, which inhibited the NH4+-induced accumulation of mRNA. These results suggest that the OKA-sensitive protein phosphatase is involved in the regulation of NADH-GOGAT gene expression and probably plays a role in the signal transduction pathway downstream from NH4+, although a signal transduction pathway other than that of nitrogen sensing could be responsible. Nuclear run-on assays demonstrated that the accumulation of NADH-GOGAT mRNA induced by the supply of either NH4+ or OKA was mainly regulated at the transcription level. OKA effects were synergistic to the NH4+-induced expression of the NADH-GOGAT gene. In the presence of K-252a, a protein kinase inhibitor, the accumulation of NADH-GOGAT mRNA induced by either NH4+ or OKA was reduced. The possible roles of protein phosphatases in the regulation of NADH-GOGAT gene expression are discussed.