The crystal structure of NGO0477 from Neisseria gonorrhoeae reveals a novel protein fold incorporating a helix-turn-helix motif

Chasing the structural diversity of the transcription regulator Mycobacterium tuberculosis HigA2

Transcription factors are the primary regulators of gene expression and recognize specific DNA sequences under diverse physiological conditions. Although they are vital for many important cellular processes, it remains unclear when and how transcription factors and DNA interact. The antitoxin from a toxin-antitoxin system is an example of negative transcriptional autoregulation: during expression of the cognate toxin it is suppressed through binding to a specific DNA sequence. In the present study, the antitoxin HigA2 from Mycobacterium tuberculosis M37Rv was structurally examined. The crystal structure of M. tuberculosis HigA2 comprises three sections: an N-terminal autocleavage region, an α-helix bundle which contains an HTH motif, and a C-terminal β-lid. The N-terminal region is responsible for toxin binding, but was shown to cleave spontaneously in its absence. The HTH motif performs a key role in DNA binding, with the C-terminal β-lid influencing the interaction by mediating the distance between the motifs. However, M. tuberculosis HigA2 exhibits a unique coordination of the HTH motif and no DNA-binding activity is detected. Three crystal structures of M. tuberculosis HigA2 show a flexible alignment of the HTH motif, which implies that the motif undergoes structural rearrangement to interact with DNA. This study reveals the molecular mechanisms of how transcription factors interact with partner proteins or DNA.

Uses of phage display in agriculture: sequence analysis and comparative modeling of late embryogenesis abundant client proteins suggest protein-nucleic acid binding functionality

A group of intrinsically disordered, hydrophilic proteins-Late Embryogenesis Abundant (LEA) proteins-has been linked to survival in plants and animals in periods of stress, putatively through safeguarding enzymatic function and prevention of aggregation in times of dehydration/heat. Yet despite decades of effort, the molecular-level mechanisms defining this protective function remain unknown. A recent effort to understand LEA functionality began with the unique application of phage display, wherein phage display and biopanning over recombinant Seed Maturation Protein homologs from Arabidopsis thaliana and Glycine max were used to retrieve client proteins at two different temperatures, with one intended to represent heat stress. From this previous study, we identified 21 client proteins for which clones were recovered, sometimes repeatedly. Here, we use sequence analysis and homology modeling of the client proteins to ascertain common sequence and structural properties that may contribute to binding affinity with the protective LEA protein. Our methods uncover what appears to be a predilection for protein-nucleic acid interactions among LEA client proteins, which is suggestive of subcellular residence. The results from this initial computational study will guide future efforts to uncover the protein protective mechanisms during heat stress, potentially leading to phage-display-directed evolution of synthetic LEA molecules.

Neisseria conserved hypothetical protein DMP12 is a DNA mimic that binds to histone-like HU protein

DNA mimic proteins are unique factors that control the DNA-binding activity of target proteins by directly occupying their DNA-binding sites. To date, only a few DNA mimic proteins have been reported and their functions analyzed. Here, we present evidence that the Neisseria conserved hypothetical protein DMP12 should be added to this list. Our gel filtration and analytical ultracentrifugation results showed that the DMP12 monomer interacts with the dimeric form of the bacterial histone-like protein HU. Subsequent structural analysis of DMP12 showed that the shape and electrostatic surface of the DMP12 monomer are similar to those of the straight portion of the bent HU-bound DNA and complementary to those of HU protein dimer. DMP12 also protects HU protein from limited digestion by trypsin and enhances the growth rate Escherichia coli. Functionally, HU proteins participate in bacterial nucleoid formation, as well as recombination, gene regulation and DNA replication. The interaction between DMP12 and HU protein might, therefore, play important roles in these DNA-related mechanisms.

Neisseria conserved protein DMP19 is a DNA mimic protein that prevents DNA binding to a hypothetical nitrogen-response transcription factor

DNA mimic proteins occupy the DNA binding sites of DNA-binding proteins, and prevent these sites from being accessed by DNA. We show here that the Neisseria conserved hypothetical protein DMP19 acts as a DNA mimic. The crystal structure of DMP19 shows a dsDNA-like negative charge distribution on the surface, suggesting that this protein should be added to the short list of known DNA mimic proteins. The crystal structure of another related protein, NHTF (Neisseria hypothetical transcription factor), provides evidence that it is a member of the xenobiotic-response element (XRE) family of transcriptional factors. NHTF binds to a palindromic DNA sequence containing a 5'-TGTNAN(11)TNACA-3' recognition box that controls the expression of an NHTF-related operon in which the conserved nitrogen-response protein [i.e. (Protein-PII) uridylyltransferase] is encoded. The complementary surface charges between DMP19 and NHTF suggest specific charge-charge interaction. In a DNA-binding assay, we found that DMP19 can prevent NHTF from binding to its DNA-binding sites. Finally, we used an in situ gene regulation assay to provide evidence that NHTF is a repressor of its down-stream genes and that DMP19 can neutralize this effect. We therefore conclude that the interaction of DMP19 and NHTF provides a novel gene regulation mechanism in Neisseria spps.