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Science  08 Mar 2002:
Vol. 295, Issue 5561, pp. 1795-1797
DOI: 10.1126/science.295.5561.1795e

Cellular nitrogen is a critical component for microbial growth. Many bacteria can use ammonium ions as a nitrogen source and have developed a sophisticated apparatus to coordinate gene expression and enzyme activities. Previous work has shown that the PII protein (encoded by glnB), in addition to its global effects on expression via the ntr genes, serves as an enzyme regulator. Unmodified PII stimulates adenylyl transferase (glnE) to modify glutamine synthetase (glnA), inhibiting its enzymatic activity when ammonium is abundant. In contrast, when glutamine levels decline (signaling nitrogen deprivation), PII itself is subject to uridylylation [by uridylyltransferase (glnD)] and induces a deadenylylation and reactivation of the synthetase.

Coutts et al. have expanded this network by showing that another member of the PII family, GlnK, is also subject to uridylylation. Unmodified GlnK binds to and inhibits the high-affinity ammonium transporter AmtB; when uridylylated (under low nitrogen conditions), GlnK is released from the membrane. This translocation to the cytoplasmic compartment has two consequences: (i) AmtB becomes an active scavenger of environmental ammonium, and (ii) GlnK relieves the inhibition of NifA (the transcriptional activator of the nitrogen fixation or nif genes) by NifL, likely through a direct interaction as suggested by Little et al. and Rudnick et al. — GJC

EMBO J.21, 536 (2002); J. Biol. Chem., 10.1071/jbc.M112262200 (2002); J. Bacteriol.184, 812 (2002).

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