Crystal Structure of Fad35R from Mycobacterium tuberculosis H37Rv in the Apo-State

A DNA-Binding Protein Tunes Septum Placement during Bacillus subtilis Sporulation

Bacillus subtilis is a bacterium capable of differentiating into a spore form more resistant to environmental stress. Early in sporulation, each cell possesses two copies of a circular chromosome. A polar FtsZ ring (Z ring) directs septation over one of the chromosomes, generating two cell compartments. The smaller "forespore" compartment initially contains only 25 to 30% of one chromosome, and this transient genetic asymmetry is required for differentiation. Timely assembly of polar Z rings and precise capture of the chromosome in the forespore both require the DNA-binding protein RefZ. To mediate its role in chromosome capture, RefZ must bind to specific DNA motifs (RBMs) that localize near the poles at the time of septation. Cells artificially induced to express RefZ during vegetative growth cannot assemble Z rings, an effect that also requires DNA binding. We hypothesized that RefZ-RBM complexes mediate precise chromosome capture by modulating FtsZ function. To investigate, we isolated 10 RefZ loss-of-function (rLOF) variants unable to inhibit cell division yet still capable of binding RBMs. Sporulating cells expressing the rLOF variants in place of wild-type RefZ phenocopied a ΔrefZ mutant, suggesting that RefZ acts through an FtsZ-dependent mechanism. The crystal structure of RefZ was solved, and wild-type RefZ and the rLOF variants were further characterized. Our data suggest that RefZ's oligomerization state and specificity for the RBMs are critical determinants influencing RefZ's ability to affect FtsZ dynamics. We propose that RBM-bound RefZ complexes function as a developmentally regulated nucleoid occlusion system for fine-tuning the position of the septum relative to the chromosome during sporulation.IMPORTANCE The bacterial nucleoid forms a large, highly organized structure. Thus, in addition to storing the genetic code, the nucleoid harbors positional information that can be leveraged by DNA-binding proteins to spatially constrain cellular activities. During B. subtilis sporulation, the nucleoid undergoes reorganization, and the cell division protein FtsZ assembles polarly to direct septation over one chromosome. The TetR family protein RefZ binds DNA motifs (RBMs) localized near the poles at the time of division and is required for both timely FtsZ assembly and precise capture of DNA in the future spore compartment. Our data suggest that RefZ exploits nucleoid organization by associating with polarly localized RBMs to modulate the positioning of FtsZ relative to the chromosome during sporulation.

The archaeal LDH-like malate dehydrogenase from Ignicoccus islandicus displays dual substrate recognition, hidden allostery and a non-canonical tetrameric oligomeric organization

The NAD(P)-dependent malate dehydrogenases (MalDHs) and NAD-dependent lactate dehydrogenases (LDHs) are homologous enzymes involved in central metabolism. They display a common protein fold and the same catalytic mechanism, yet have a stringent capacity to discriminate between their respective substrates. The MalDH/LDH superfamily is divided into several phylogenetically related groups. It has been shown that the canonical LDHs and LDH-like group of MalDHs are primarily tetrameric enzymes that diverged from a common ancestor. In order to gain understanding of the evolutionary history of the LDHs and MalDHs, the biochemical properties and crystallographic structure of the LDH-like MalDH from the hyperthermophilic archaeon Ignicoccus islandicus (I. isl) were determined. I. isl MalDH recognizes oxaloacetate as main substrate, but it is also able to use pyruvate. Surprisingly, with pyruvate, the enzymatic activity profile looks like that of allosteric LDHs, suggesting a hidden allosteric capacity in a MalDH. The I. isl MalDH tetrameric structure in the apo state is considerably different from those of canonical LDH-like MalDHs and LDHs, representing an alternative oligomeric organization. A comparison with MalDH and LDH counterparts provides strong evidence that the divergence between allosteric and non-allosteric members of the superfamily involves homologs with intermediate, atypical properties.

Loop-to-helix transition in the structure of multidrug regulator AcrR at the entrance of the drug-binding cavity

Multidrug transcription regulator AcrR from Salmonella enterica subsp. enterica serovar Typhimurium str. LT2 belongs to the tetracycline repressor family, one of the largest groups of bacterial transcription factors. The crystal structure of dimeric AcrR was determined and refined to 1.56Å resolution. The tertiary and quaternary structures of AcrR are similar to those of its homologs. The multidrug binding site was identified based on structural alignment with homologous proteins and has a di(hydroxyethyl)ether molecule bound. Residues from helices α4 and α7 shape the entry into this binding site. The structure of AcrR reveals that the extended helical conformation of helix α4 is stabilized by the hydrogen bond between Glu67 (helix α4) and Gln130 (helix α7). Based on the structural comparison with the closest homolog structure, the Escherichia coli AcrR, we propose that this hydrogen bond is responsible for control of the loop-to-helix transition within helix α4. This local conformational switch of helix α4 may be a key step in accessing the multidrug binding site and securing ligands at the binding site. Solution small-molecule binding studies suggest that AcrR binds ligands with their core chemical structure resembling the tetracyclic ring of cholesterol.