A role for the signal transduction protein P(II) in the control of nitrate/nitrite uptake in a cyanobacterium

Abstract

In the cyanobacterium Synechococcus sp. strain PCC 7942, ammonium exerts a rapid and reversible inhibition of the nitrate and nitrite uptake, and the P(II) protein (GlnB) is differentially phosphorylated depending on the intracellular N/C balance, RNA/DNA hybridizations, as well as nitrate and nitrite uptake experiments, were carried out with the wild-type strain and a P(II)-null mutant. The transcriptional control by ammonium of the expression of the nir-nrtABCD-narB operon remained operative in the mutant but, in contrast to the mild-type strain, the mutant took up nitrate and nitrite even in the presence of ammonium. However, the wild-type phenotype was restored by insertion of a copy of the wild-type glnB gene in the genome of the P(II)-null mutant. These results indicate that the unphosphorylntcd form of P(II) is involved in the short-term inhibition by ammonium of the nitrate and nitrite uptake in Synechococcus sp. strain PCC 7942.