Genome organization in prokaryotes

Epigenetic modulator UVI5008 inhibits MRSA by interfering with bacterial gyrase

The impact of multi-drug resistant bacterial strains on human health is reaching worrisome levels. Over 2 million people are infected by resistant bacteria, and more than 700,000 people die each year because of the continuous spread of resistant strains. The development of new antibiotics and the prudent use of existing ones to prolong their lifespan require a constant effort by drug industries and healthcare workers. The re-purposing of existing drugs for use as antimicrobial agents would streamline the development of new antibacterial strategies. As part of this effort, we screened a panel of drugs previously characterized to be epigenetic modulators/pro-apoptotic/differentiative drugs. We selected a few compounds that alter Gram-positive growth. Among these, UVI5008, a derivative of the natural compound psammaplin A (Psa_A), was identified. The interaction of Psa_A with the DNA gyrase enzyme has been shown, and here, we hypothesized and confirmed the gyrase-specific activity by biochemical assays. UVI5008 exhibited growth inhibition activity against Staphylococcus aureus via structural modification of the cell wall, which was observed by SEM electron microscopy. Based on our findings, we propose UVI5008 as an alternative antibacterial compound against methicillin-resistant (Met.R) S. aureus strains.

Rhizobium plasmids in bacteria-legume interactions

The functional analysis of plasmids in Rhizobium strains has concentrated mainly on the symbiotic plasmid (pSym). However, genetic information relevant to both symbiotic and saprophytic Rhizobium life cycles, localized on other 'cryptic' replicons, has also been reported. Information is reviewed which concerns functional features encoded in plasmids other than the pSym: biosynthesis of cell surface polysaccharides, metabolic processes, the utilization of plant exudates, aromatic compounds and diverse sugars, and features involved symbiotic performance. In addition, factors which affect plasmid evolution through their influence on structural features of the plasmids, such as conjugative transfer and genomic rearrangements, is discussed. Based on the overall data, we propose that together the plasmids and the chromosome constitute a fully integrated genomic complex, entailing structural features as well as saprophytic and cellular functions.

Global genomic arrangement of bacterial genes is closely tied with the total transcriptional efficiency

The availability of a large number of sequenced bacterial genomes allows researchers not only to derive functional and regulation information about specific organisms but also to study the fundamental properties of the organization of a genome. Here we address an important and challenging question regarding the global arrangement of operons in a bacterial genome: why operons in a bacterial genome are arranged in the way they are. We have previously studied this question and found that operons of more frequently activated pathways tend to be more clustered together in a genome. Specifically, we have developed a simple sequential distance-based pseudo energy function and found that the arrangement of operons in a bacterial genome tend to minimize the clusteredness function (C value) in comparison with artificially-generated alternatives, for a variety of bacterial genomes. Here we extend our previous work, and report a number of new observations: (a) operons of the same pathways tend to group into a few clusters rather than one; and (b) the global arrangement of these operon clusters tend to minimize a new “energy” function (C⁺ value) that reflects the efficiency of the transcriptional activation of the encoded pathways. These observations provide insights into further study of the genomic organization of genes in bacteria.

Identification of replication origins in prokaryotic genomes

The availability of hundreds of complete bacterial genomes has created new challenges and simultaneously opportunities for bioinformatics. In the area of statistical analysis of genomic sequences, the studies of nucleotide compositional bias and gene bias between strands and replichores paved way to the development of tools for prediction of bacterial replication origins. Only a few (about 20) origin regions for eubacteria and archaea have been proven experimentally. One reason for that may be that this is now considered as an essentially bioinformatics problem, where predictions are sufficiently reliable not to run labor-intensive experiments, unless specifically needed. Here we describe the main existing approaches to the identification of replication origin (oriC) and termination (terC) loci in prokaryotic chromosomes and characterize a number of computational tools based on various skew types and other types of evidence. We also classify the eubacterial and archaeal chromosomes by predictability of their replication origins using skew plots. Finally, we discuss possible combined approaches to the identification of the oriC sites that may be used to improve the prediction tools, in particular, the analysis of DnaA binding sites using the comparative genomic methods.

Prediction, identification, and artificial selection of DNA rearrangements in Rhizobium: toward a natural genomic design

Based on the DNA sequence of the symbiotic plasmid of Rhizobium strain NGR234, we predicted potential rearrangements generated by homologous recombination. All predicted rearrangements were identified experimentally by using a PCR-based methodology. Thus, the predicted and the actual dynamic maps of the replicon coincide. By using an approach that does not involve the introduction of exogenous genetic elements, derivative populations that are pure for specific rearrangements were obtained. We propose that knowledge of the DNA sequence of a genome offers the possibility of designing pathways of sequential rearrangements leading to alternative genomic structures. An experimental strategy to isolate bacterial populations containing the desired structures is discussed.

The diverse and dynamic structure of bacterial genomes

Bacterial genome sizes, which range from 500 to 10,000 kbp, are within the current scope of operation of large-scale nucleotide sequence determination facilities. To date, 8 complete bacterial genomes have been sequenced, and at least 40 more will be completed in the near future. Such projects give wonderfully detailed information concerning the structure of the organism's genes and the overall organization of the sequenced genomes. It will be very important to put this incredible wealth of detail into a larger biological picture: How does this information apply to the genomes of related genera, related species, or even other individuals from the same species? Recent advances in pulsed-field gel electrophoretic technology have facilitated the construction of complete and accurate physical maps of bacterial chromosomes, and the many maps constructed in the past decade have revealed unexpected and substantial differences in genome size and organization even among closely related bacteria. This review focuses on this recently appreciated plasticity in structure of bacterial genomes, and diversity in genome size, replicon geometry, and chromosome number are discussed at inter- and intraspecies levels.

Gene amplification and genomic plasticity in prokaryotes

Gene amplification is a common feature of the genome of prokaryotic organisms. In this review, we analyze different instances of gene amplification in a variety of prokaryotes, including their mechanisms of generation and biological role. Growing evidence supports the concept that gene amplification be considered not as a mutation but rather as a dynamic genomic state related to the adaptation of bacterial populations to changing environmental conditions or biological interactions. In this context, the potentially amplifiable DNA regions impose a defined dynamic structure on the genome. If such structure has indeed been selected during evolution, it is a particularly challenging hypothesis.

Organization of the leading region of IncN plasmid pKM101 (R46): a regulation controlled by CUP sequence elements

Analysis of the nucleotide sequence of the 13.8 kb leading region of the IncN plasmid pKM101 (a deletion derivative of R46) revealed eight copies of highly conserved repetitive elements, CUP (Conserved UPstream), and at least nine novel open reading frames (ORFs). Appropriate protein products were identified for eight ORFs and the analysis of their deduced amino acid sequences revealed similarities with some well-known proteins (KorA of RK2/RP4, RecX and PsiB) that may play a role in the adaptation of promiscuous plasmids to the new host. Comparison of CUP elements revealed that the CUP core is 417 nucleotides long and consists of two portions that markedly differ in GC content. The larger portion (307 nucleotides) of the core is about 74% GC and contains at least one NotI site, while the other (110 nucleotides) is only about 40% GC. The remarkable features of CUP elements is that five of them are oriented in the same direction and fused in a similar mode to the open reading frames (ORFs) that are able to encode unrelated proteins. The spacings between the right boundary of the CUP core and the potential ATG start codons of these ORFs are slightly different in length (16 to 18 bp), highly divergent in sequence but in all cases contain the conserved hexamer 5'-AGGAGT-3' at the position that is typical for the ribosome binding site of Escherichia coli. The A+T-rich portion of the CUP sequences contains the strong negatively regulated promoter and appears to function as a genetic switch that coordinately controls the expression of CUP-fused genes during the conjugal transfer. These findings suggest that seven plasmid genes fused to the CUP elements including repA and two ard genes encoding positively acting replication protein and antirestriction proteins, respectively, may be members of one regulatory network based on the CUP elements and two plasmid-encoded regulatory proteins ArdK and ArdR. At least, the ArdK protein may act as a typical repressor by binding to the promoter region of the CUP sequence. Most of the structural and functional features of organization of the CUP-controlled regulatory network are associated with the idea that the CUP elements may be involved in the natural genetic engineering process of organizing various functionally related genes in one regulon.

Head-on collision between a DNA replication apparatus and RNA polymerase transcription complex

An in vitro system reconstituted from purified proteins has been used to examine what happens when the DNA replication apparatus of bacteriophage T4 collides with an Escherichia coli RNA polymerase ternary transcription complex that is poised to move in the direction opposite to that of the moving replication fork. In the absence of a DNA helicase, the replication fork stalls for many minutes after its encounter with the RNA polymerase. However, when the T4 gene 41 DNA helicase is present, the replication fork passes the RNA polymerase after a pause of a few seconds. This brief pause is longer than the pause observed for a codirectional collision between the same two polymerases, suggesting that there is an inherent disadvantage to having replication and transcription directions oriented head to head. As for a codirectional collision, the RNA polymerase remains competent to resume faithful RNA chain elongation after the DNA replication fork passes; most strikingly, the RNA polymerase has switched from its original template strand to use the newly synthesized daughter DNA strand as the template.