Studies on the roles of GlnK and GlnB in regulating Klebsiella pneumoniae NifL-dependent nitrogen control

In Klebsiella pneumoniae, nitrogen fixation (nif) genes are regulated in re sponse to fixed nitrogen and oxygen. The activity of the nif-specific trans criptional activator NifA is modulated by NifL, which mediates both oxygen and nitrogen control. The signal transduction protein GlnK is required to r elieve the inhibitory effect of NifL an NifA that occurs when the intracell ular N status is high and in a wild-type cell, the action of GlnK cannot be substituted by the structurally related protein PII. We have studied the m odulation of NifA activity by NifL in an heterologous system in which the h ost organism is Escherichia coil. Using a Delta glnB,Delta glnK mutant, we have shown that the modulation of NifA activity by NifL is dependent on the concentration of GlnK in the cell and that when overproduced, PII can subs titute for GlnK. Furthermore, our data suggest that PII can counteract the positive action of GlnK in relieving NifL-dependent inhibition of NifA acti vity. This negative effect of PII may be physiologically important in estab lishing repression of nif gene expression when the intracellular nitrogen s tatus rises. (C) 1999 Federation of European Microbiological Societies. Pub lished by Elsevier Science B.V. All rights reserved.