Summary The role of theKlebsiella pneumoniae P_II protein (encoded byglnB) in nitrogen regulation has been studied using two classes ofglnB mutants. In Class I mutants P_II appears not to be uridylylated in nitrogen-limiting conditions and in Class II mutants P_II is not synthesised. The effects of these mutations on expression from nitrogen-regulated promoters indicate that P_II is not absolutely required for nitrogen control. Furthermore the uridylylated form of P_II(P_II-UMP) plays a significant role in the response to changes in nitrogen status by counteracting the effect of P_II on NtrB-mediated dephosphorylation of NtrC. P_II is not involved in thenif-specific response to changes in nitrogen status mediated by NifL.