Escherichia coli AmtB is a member of the MEP/Amt family of ammonia transporters found in archaea, eubacteria, fungi, plants and animals. In prokaryotes, AmtB homologues are co-transcribed with a PII paralogue, GlnK, in response to nitrogen limitation. Here, we show that AmtB antagonizes PII signalling through NRII and that co-expression of GlnK with AmtB overcomes this antagonism. In cells lacking GlnK, expression of AmtB during nitrogen starvation prevented deinduction of Ntr gene expression when a nitrogen source became available. The absence of AmtB in cells lacking GlnK allowed rapid reduction of Ntr gene expression during this transition, indicating that one function of GlnK is to prevent AmtB-mediated antagonism of PII signalling after nitrogen starvation. Other roles of GlnK in controlling Ntr gene expression and maintaining viability during nitrogen starvation were unaffected by AmtB. Expression of AmtB from a constitutive promoter under nitrogen-rich conditions induced full expression of glnALG and elevated expression of glnK in wild-type and glnK cells; thus, the ability of AmtB to raise Ntr gene expression did not require a factor found only in nitrogen-starved cells. Experiments with intact cells showed that AmtB acted downstream of a uridylyl transferase uridylyl-removing enzyme (UTase/UR) and upstream of NRII, suggesting that the target was PII. AmtB also slowed the deuridylylation of PII approximately UMP upon ammonia addition, showing that multiple PII interactions were affected by AmtB. Our data are consistent with a hypothesis that AmtB interacts with PII and GlnK, and that co-transcription of glnK and amtB prevents titration of PII when AmtB is highly expressed.