Role of PII proteins in nitrogen fixation control of Herbaspirillum seropedicae strain SmR1

BACKGROUND: The PII protein family comprises homotrimeric proteins which act as transducers of the cellular nitrogen and carbon status in prokaryotes and plants. In Herbaspirillum seropedicae, two PII-like proteins (GlnB and GlnK), encoded by the genes glnB and glnK, were identified. The glnB gene is monocistronic and its expression is constitutive, while glnK is located in the nlmAglnKamtB operon and is expressed under nitrogen-limiting conditions. RESULTS: In order to determine the involvement of the H. seropedicae glnB and glnK gene products in nitrogen fixation, a series of mutant strains were constructed and characterized. The glnK(- )mutants were deficient in nitrogen fixation and they were complemented by plasmids expressing the GlnK protein or an N-truncated form of NifA. The nitrogenase post-translational control by ammonium was studied and the results showed that the glnK mutant is partially defective in nitrogenase inactivation upon addition of ammonium while the glnB mutant has a wild-type phenotype. CONCLUSIONS: Our results indicate that GlnK is mainly responsible for NifA activity regulation and ammonium-dependent post-translational regulation of nitrogenase in H. seropedicae.