During transcription initiation, RNA polymerase (RNAP) binds and unwinds promoter DNA to form an RNAP-promoter open complex. We have determined crystal structures at 2.9 and 3.0 Å resolution of functional transcription initiation complexes comprising Thermus thermophilus RNA polymerase, σA, and a promoter DNA fragment corresponding to the transcription bubble and downstream dsDNA of the RNAP-promoter open complex. The structures show that σ recognizes the -10 element and discriminator element through interactions that include the unstacking and insertion into pockets of three DNA bases, and that RNAP recognizes the −4/+2 region through interactions that include the unstacking and insertion into a pocket of the +2 base. The structures further show that interactions between σ and template-strand ssDNA pre-organize template-strand ssDNA to engage the RNAP active center.

A new drug target-- the "switch region"--has been identified within bacterial RNA polymerase (RNAP), the enzyme that mediates bacterial RNA synthesis. The new target serves as the binding site for compounds that inhibit bacterial RNA synthesis and kill bacteria. Since the new target is present in most bacterial species, compounds that bind to the new target are active against a broad spectrum of bacterial species. Since the new target is different from targets of other antibacterial agents, compounds that bind to the new target are not cross-resistant with other antibacterial agents. Four antibiotocs that function through the new target have been identified: myxopyronin, corallopyronin, ripostatin, and lipiarmycin. This review summarizes the switch region, switch-region inhibitors, and implications for antibacterial drug discovery.

SUMMARY Capistruin, a ribosomally synthesized post-translationally modified peptide produced by Burkholderia thailandensis E264, efficiently inhibits growth of Burkholderia and closely related Pseudomonas strains. The functional target of capistruin is unknown. Capistruin is a threaded-lasso peptide (lariat peptide), comprising an N-terminal 9-amino-acid ring followed by a 10-amino-acid C-terminal tail that is threaded through the ring. The structure of capistruin is similar to that of microcin J25 (MccJ25), a threaded-lasso antibacterial peptide that is produced by some strains of Escherichia coli and targets DNA-dependent RNA polymerase (RNAP). Here, we show that capustruin, like MccJ25, inhibits wild-type E. coli RNAP but not mutant, MccJ25-resistant, E. coli RNAP. We show further that an E. coli strain resistant to MccJ25 due to a mutation in an RNAP subunit gene exhibits resistance to capistruin. The results indicate that the structural similarity of capistruin and MccJ25 reflects functional similarity and suggest that the functional target of capistruin, and possibly other threaded-lasso peptides, is bacterial RNAP.

Bloom syndrome is an autosomal recessive disorder caused by mutations in the RecQ family helicase BLM that is associated with growth retardation and predisposition to cancer. BLM helicase has a high specificity for non-canonical G-quadruplex (G4) DNA structures, which are formed by G-rich DNA strands and play an important role in the maintenance of genomic integrity. Here, we used single-molecule FRET to define the mechanism of interaction of BLM helicase with intra-stranded G4 structures. We show that the activity of BLM is substrate dependent, and highly regulated by a short ssDNA segment that separates the G4 motif from dsDNA. We demonstrate cooperativity between the RQC and HRDC domains of BLM during binding and unfolding of the G4 structure, where the RQC domain interaction with G4 is stabilized by HRDC binding to ssDNA. We present a model that proposes a unique role for G4 structures in modulating the activity of DNA processing enzymes.

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The initial step of viral infection is the binding of a virus onto the host cell surface. This first viral-host interaction would determine subsequent infection steps and the fate of the entire infection process. A basic understating of the underlining mechanism of initial virus-host binding is a prerequisite for establishing the nature of viral infection. Bacteriophage λ and its host Escherichia coli serve as an excellent paradigm for this purpose. λ phages bind to specific receptors, LamB, on the host cell surface during the infection process. The interaction of bacteriophage λ with the LamB receptor has been the topic of many studies, resulting in wealth of information on the structure, biochemical properties and molecular biology of this system. Recently, imaging studies using fluorescently labeled phages and its receptor unveil the role of spatiotemporal dynamics and divulge the importance of stochasticity from hidden variables in the infection outcomes. The scope of this article is to review the present state of research on the interaction of bacteriophage λ and its E. coli receptor, LamB.

Background: Homologous recombination is regulated both positively and negatively in eukaryotic cells to suppress genomic instability. Results: FBH1 can disrupt RAD51 filaments in vitro and suppresses formation of spontaneous RAD51 foci in mammalian cells. In cells defective for FBH1, hyper-recombination is observed. Conclusion: FBH1 is a negative regulator of homologous recombination. Significance: RAD51 activity must be carefully controlled to preserve genomic integrity.

Sulfur oxidation in Pseudaminobacter salicylatoxidans KCT001 is rendered by the combined action of several enzymes encoded by a thiosulfate-inducible sox operon. In this study it has been conclusively demonstrated by insertional mutagenesis that the regulatory gene of this operon is soxR, which encodes a DNA-binding protein belonging to the ArsR-SmtB family. SoxR was found to bind to two promoter-operator segments within the sox cluster, of which the one (wx) located between soxW and soxX controls the expression of sulfur-oxidation genes soxX through soxD while the other, a bi-directional element (sv) located between soxS and soxV, controls the expression of soxVW in one direction and the putative regulatory cluster soxSRT in the other. In the case of the wx promoter the repressor was found to bind in a cooperative manner to two distinct binding sites having different affinities, while in the case of the sv promoter binding occurred at a symmetric dimeric site and involved a higher degree of cooperativity. The high degree of cooperativity observed in the binding of SoxR to its target sites seemed to be due to the propensity of SoxR monomers to form dimers. The apparent dissociation constants of the SoxR-operator complexes were in the nanomolar range, indicating relatively strong interactions. It was demonstrated using a reporter system in Escherichia coli that this high-affinity binding of SoxR led to efficient repression in trans. Thus the role of SoxR as a repressor of the sox operon has not only been conclusively established but it has also been shown that this repression is brought about through cooperative interactions of SoxR with dimeric binding sites that occlude the operon promoters.