SummaryHMm is the only archaeal histone in Methanosarcina mazei Göl and recombinant HMm, synthesized by expression of MM1825 in Escherichia coli, has been purified and confirmed to have the DNA binding and compaction properties characteristic of an archaeal histone. Insertion of a puromycin resistance conferring cassette (pac) into MM1825 was not lethal but resulted in mutants (M. mazei MM1825::pac) that have impaired ability to grow on methanol and trimethylamine. Loss of HMm also resulted in increased sensitivity to UV light and decreased transcript levels for~25% of all M. mazei genes. For most genes, the transcript decrease was 3-to 10-fold, but transcripts of MM483 (small heat-shock protein), MM1688 (trimethylamine:corrinoid methyl transferase) and MM3195 (transcription regulator), were reduced 100-, 100-and 25-fold, respectively, in M. mazei MM1825::pac cells. Transcripts of only five adjacent genes that appear to constitute an aromatic amino acid biosynthetic operon were elevated in M. mazei MM1825::pac cells. Complementary synthesis of HMm from a plasmid transformed into M. mazei MM1825::pac restored wild-type growth and transcript levels.

In the diazotroph Klebsiella pneumoniae, the nitrogen sensory protein GlnK mediates the cellular nitrogen status towards the NifL/NifA system that regulates transcription of the nitrogen fixation genes in response to ammonium and molecular oxygen. To identify amino acids of GlnK essential for this signal transduction by protein–protein interaction, we performed random point mutagenesis by PCR amplification under conditions of reduced Taq polymerase fidelity. Three thousand two hundred mutated glnK genes were screened to identify those that would no longer complement a K. pneumoniaeΔglnK strain for growth under nitrogen fixing conditions. Twenty‐four candidates resulting in a Nif− phenotype were identified, carrying 1–11 amino acid changes in GlnK. Based on these findings, as well as structural data, several single mutations were introduced into glnK by site‐directed mutagenesis, and the Nif phenotype and the respective effects on NifA‐mediated nif gene induction was monitored in K. pneumoniae using a chromosomal nifK′–′lacZ fusion. Single amino acid changes resulting in significant nif gene inhibition under nitrogen limiting conditions were located within the highly conserved T‐loop (A43G, A49T and N54D), the body of the protein (G87V and K79E) and in the C‐terminal region (I100M, R103S, E106Q and D108G). Complex formation analyses between GlnK (wild‐type or derivatives) and NifL or NifA in response to 2‐oxoglutarate indicated that: (a) besides the T‐loop, the C‐terminal region of GlnK is essential for the interaction with NifL and NifA and (b) GlnK binds both proteins in the absence of 2‐oxoglutarate, whereas, in the presence of 2‐oxoglutarate, NifA is released but NifL remains bound to GlnK.

In Klebsiella pneumoniae nitrogen fixation is tightly controlled in response to ammonium and molecular oxygen by the NifL/NifA regulatory system. Under repressing conditions, NifL inhibits the nif-specific transcriptional activator NifA by direct protein-protein interaction, whereas under anaerobic and nitrogen-limited conditions sequestration of reduced NifL to the cytoplasmic membrane impairs inhibition of cytoplasmic NifA by NifL. We report here on a genetic screen to identify amino acids of NifL essential for sequestration to the cytoplasmic membrane under nitrogen-fixing conditions. Overall, 11,500 mutated nifL genes of three independently generated pools were screened for those conferring a Nif ؊ phenotype. Based on the respective amino acid changes of nonfunctional derivatives obtained in the screen, and taking structural data into account as well, several point mutations were introduced into nifL by site-directed mutagenesis. The majority of amino acid changes resulting in a significant nif gene inhibition were located in the N-terminal domain (N46D, Q57L, Q64R, N67S, N69S, R80C, and W87G) and the Q-linker (K271E). Further analyses demonstrated that positions N69, R80, and W87 are essential for binding the FAD cofactor, whereas primarily Q64 and N46, but also Q57 and N67, appear to be crucial for direct membrane contact of NifL under oxygen and nitrogen limitation. Based on these findings, we propose that those four amino acids most likely located on the protein surface, as well as the presence of the FAD cofactor, are crucial for the correct overall protein conformation and respective surface charge, allowing NifL sequestration to the cytoplasmic membrane under derepressing conditions.