Visualizing the proteome of Escherichia coli: an efficient and versatile method for labeling chromosomal coding DNA sequences (CDSs) with fluorescent protein genes

Novel molecular requirements for CRISPR RNA-guided transposition

CRISPR-associated transposases (CASTs) direct DNA integration downstream of target sites using the RNA-guided DNA binding activity of nuclease-deficient CRISPR-Cas systems. Transposition relies on several key protein-protein and protein-DNA interactions, but little is known about the explicit sequence requirements governing efficient transposon DNA integration activity. Here, we exploit pooled library screening and high-throughput sequencing to reveal novel sequence determinants during transposition by the Type I-F Vibrio cholerae CAST system (VchCAST). On the donor DNA, large transposon end libraries revealed binding site nucleotide preferences for the TnsB transposase, as well as an additional conserved region that encoded a consensus binding site for integration host factor (IHF). Remarkably, we found that VchCAST requires IHF for efficient transposition, thus revealing a novel cellular factor involved in CRISPR-associated transpososome assembly. On the target DNA, we uncovered preferred sequence motifs at the integration site that explained previously observed heterogeneity with single-base pair resolution. Finally, we exploited our library data to design modified transposon variants that enable in-frame protein tagging. Collectively, our results provide new clues about the assembly and architecture of the paired-end complex formed between TnsB and the transposon DNA, and inform the design of custom payload sequences for genome engineering applications with CAST systems.

Single-Cell Technologies to Study Phenotypic Heterogeneity and Bacterial Persisters

Antibiotic persistence is a phenomenon in which rare cells of a clonal bacterial population can survive antibiotic doses that kill their kin, even though the entire population is genetically susceptible. With antibiotic treatment failure on the rise, there is growing interest in understanding the molecular mechanisms underlying bacterial phenotypic heterogeneity and antibiotic persistence. However, elucidating these rare cell states can be technically challenging. The advent of single-cell techniques has enabled us to observe and quantitatively investigate individual cells in complex, phenotypically heterogeneous populations. In this review, we will discuss current technologies for studying persister phenotypes, including fluorescent tags and biosensors used to elucidate cellular processes; advances in flow cytometry, mass spectrometry, Raman spectroscopy, and microfluidics that contribute high-throughput and high-content information; and next-generation sequencing for powerful insights into genetic and transcriptomic programs. We will further discuss existing knowledge gaps, cutting-edge technologies that can address them, and how advances in single-cell microbiology can potentially improve infectious disease treatment outcomes.

λ Recombination and Recombineering

The bacteriophage λ Red homologous recombination system has been studied over the past 50 years as a model system to define the mechanistic details of how organisms exchange DNA segments that share extended regions of homology. The λ Red system proved useful as a system to study because recombinants could be easily generated by co-infection of genetically marked phages. What emerged from these studies was the recognition that replication of phage DNA was required for substantial Red-promoted recombination in vivo, and the critical role that double-stranded DNA ends play in allowing the Red proteins access to the phage DNA chromosomes. In the past 16 years, however, the λ Red recombination system has gained a new notoriety. When expressed independently of other λ functions, the Red system is able to promote recombination of linear DNA containing limited regions of homology (∼50 bp) with the Escherichia coli chromosome, a process known as recombineering. This review explains how the Red system works during a phage infection, and how it is utilized to make chromosomal modifications of E. coli with such efficiency that it changed the nature and number of genetic manipulations possible, leading to advances in bacterial genomics, metabolic engineering, and eukaryotic genetics.

Proteome-wide subcellular topologies of E. coli polypeptides database (STEPdb)

Cell compartmentalization serves both the isolation and the specialization of cell functions. After synthesis in the cytoplasm, over a third of all proteins are targeted to other subcellular compartments. Knowing how proteins are distributed within the cell and how they interact is a prerequisite for understanding it as a whole. Surface and secreted proteins are important pathogenicity determinants. Here we present the STEP database (STEPdb) that contains a comprehensive characterization of subcellular localization and topology of the complete proteome of Escherichia coli. Two widely used E. coli proteomes (K-12 and BL21) are presented organized into thirteen subcellular classes. STEPdb exploits the wealth of genetic, proteomic, biochemical, and functional information on protein localization, secretion, and targeting in E. coli, one of the best understood model organisms. Subcellular annotations were derived from a combination of bioinformatics prediction, proteomic, biochemical, functional, topological data and extensive literature re-examination that were refined through manual curation. Strong experimental support for the location of 1553 out of 4303 proteins was based on 426 articles and some experimental indications for another 526. Annotations were provided for another 320 proteins based on firm bioinformatic predictions. STEPdb is the first database that contains an extensive set of peripheral IM proteins (PIM proteins) and includes their graphical visualization into complexes, cellular functions, and interactions. It also summarizes all currently known protein export machineries of E. coli K-12 and pairs them, where available, with the secretory proteins that use them. It catalogs the Sec- and TAT-utilizing secretomes and summarizes their topological features such as signal peptides and transmembrane regions, transmembrane topologies and orientations. It also catalogs physicochemical and structural features that influence topology such as abundance, solubility, disorder, heat resistance, and structural domain families. Finally, STEPdb incorporates prediction tools for topology (TMHMM, SignalP, and Phobius) and disorder (IUPred) and implements the BLAST2STEP that performs protein homology searches against the STEPdb.

Phage recombinases and their applications

The homologous recombination systems of linear double-stranded (ds)DNA bacteriophages are required for the generation of genetic diversity, the repair of dsDNA breaks, and the formation of concatemeric chromosomes, the immediate precursor to packaging. These systems have been studied for decades as a means to understand the basic principles of homologous recombination. From the beginning, it was recognized that these recombinases are linked intimately to the mechanisms of phage DNA replication. In the last decade, however, investigators have exploited these recombination systems as tools for genetic engineering of bacterial chromosomes, bacterial artificial chromosomes, and plasmids. This recombinational engineering technology has been termed "recombineering" and offers a new paradigm for the genetic manipulation of bacterial chromosomes, which is far more efficient than the classical use of nonreplicating integration vectors for gene replacement. The phage λ Red recombination system, in particular, has been used to construct gene replacements, deletions, insertions, inversions, duplications, and single base pair changes in the Escherichia coli chromosome. This chapter discusses the components of the recombination systems of λ, rac prophage, and phage P22 and properties of single-stranded DNA annealing proteins from these and other phage that have been instrumental for the development of this technology. The types of genetic manipulations that can be made are described, along with proposed mechanisms for both double-stranded DNA- and oligonucleotide-mediated recombineering events. Finally, the impact of this technology to such diverse fields as bacterial pathogenesis, metabolic engineering, and mouse genomics is discussed.

The universally conserved prokaryotic GTPases

Members of the large superclass of P-loop GTPases share a core domain with a conserved three-dimensional structure. In eukaryotes, these proteins are implicated in various crucial cellular processes, including translation, membrane trafficking, cell cycle progression, and membrane signaling. As targets of mutation and toxins, GTPases are involved in the pathogenesis of cancer and infectious diseases. In prokaryotes also, it is hard to overestimate the importance of GTPases in cell physiology. Numerous papers have shed new light on the role of bacterial GTPases in cell cycle regulation, ribosome assembly, the stress response, and other cellular processes. Moreover, bacterial GTPases have been identified as high-potential drug targets. A key paper published over 2 decades ago stated that, "It may never again be possible to capture [GTPases] in a family portrait" (H. R. Bourne, D. A. Sanders, and F. McCormick, Nature 348:125-132, 1990) and indeed, the last 20 years have seen a tremendous increase in publications on the subject. Sequence analysis identified 13 bacterial GTPases that are conserved in at least 75% of all bacterial species. We here provide an overview of these 13 protein subfamilies, covering their cellular functions as well as cellular localization and expression levels, three-dimensional structures, biochemical properties, and gene organization. Conserved roles in eukaryotic homologs will be discussed as well. A comprehensive overview summarizing current knowledge on prokaryotic GTPases will aid in further elucidating the function of these important proteins.

Functional characterization of an alkaline exonuclease and single strand annealing protein from the SXT genetic element of Vibrio cholerae

BACKGROUND

SXT is an integrating conjugative element (ICE) originally isolated from Vibrio cholerae, the bacterial pathogen that causes cholera. It houses multiple antibiotic and heavy metal resistance genes on its ca. 100 kb circular double stranded DNA (dsDNA) genome, and functions as an effective vehicle for the horizontal transfer of resistance genes within susceptible bacterial populations. Here, we characterize the activities of an alkaline exonuclease (S066, SXT-Exo) and single strand annealing protein (S065, SXT-Bet) encoded on the SXT genetic element, which share significant sequence homology with Exo and Bet from bacteriophage lambda, respectively.

RESULTS

SXT-Exo has the ability to degrade both linear dsDNA and single stranded DNA (ssDNA) molecules, but has no detectable endonuclease or nicking activities. Adopting a stable trimeric arrangement in solution, the exonuclease activities of SXT-Exo are optimal at pH 8.2 and essentially require Mn2+ or Mg2+ ions. Similar to lambda-Exo, SXT-Exo hydrolyzes dsDNA with 5'- to 3'-polarity in a highly processive manner, and digests DNA substrates with 5'-phosphorylated termini significantly more effectively than those lacking 5'-phosphate groups. Notably, the dsDNA exonuclease activities of both SXT-Exo and lambda-Exo are stimulated by the addition of lambda-Bet, SXT-Bet or a single strand DNA binding protein encoded on the SXT genetic element (S064, SXT-Ssb). When co-expressed in E. coli cells, SXT-Bet and SXT-Exo mediate homologous recombination between a PCR-generated dsDNA fragment and the chromosome, analogous to RecET and lambda-Bet/Exo.

CONCLUSIONS

The activities of the SXT-Exo protein are consistent with it having the ability to resect the ends of linearized dsDNA molecules, forming partially ssDNA substrates for the partnering SXT-Bet single strand annealing protein. As such, SXT-Exo and SXT-Bet may function together to repair or process SXT genetic elements within infected V. cholerae cells, through facilitating homologous DNA recombination events. The results presented here significantly extend our general understanding of the properties and activities of alkaline exonuclease and single strand annealing proteins of viral/bacteriophage origin, and will assist the rational development of bacterial recombineering systems.

A simple plasmid-based system that allows rapid generation of tightly controlled gene expression in Staphylococcus aureus

We have established a plasmid-based system that enables tightly controlled gene expression and the generation of GFP fusion proteins in Staphylococcus aureus simply and rapidly. This system takes advantage of an Escherichia coli–S. aureus shuttle vector that contains the replication region of the S. aureus theta-mode multiresistance plasmid pSK41, and is therefore a stable low-copy-number plasmid in the latter organism. This vector also contains a multiple cloning site downstream of the IPTG-inducible Pspac promoter for insertion of the gene of interest. Production of encoded proteins can be stringently regulated in an IPTG-dependent manner by introducing a pE194-based plasmid, pGL485, carrying a constitutively expressed lacI gene. Using GFP fusions to two essential proteins of S. aureus, FtsZ and NusA, we showed that our plasmid allowed tightly controlled gene expression and accurate localization of fusion proteins with no detrimental effect on cells at low inducer concentrations. At higher IPTG concentrations, we obtained sixfold overproduction of protein compared with wild-type levels, with FtsZ–GFP-expressing cells showing lysis and delocalized fluorescence, while NusA–GFP showed only delocalized fluorescence. These results show that our system is capable of titratable induction of gene expression for localization or overexpression studies.

Correlation between ribonucleoside-diphosphate reductase and three replication proteins in Escherichia coli

Background

There has long been evidence supporting the idea that RNR and the dNTP-synthesizing complex must be closely linked to the replication complex or replisome. We contributed to this body of evidence in proposing the hypothesis of the replication hyperstructure. A recently published work called this postulate into question, reporting that NrdB is evenly distributed throughout the cytoplasm. Consequently we were interested in the localization of RNR protein and its relationship with other replication proteins.

Results

We tagged NrdB protein with 3×FLAG epitope and detected its subcellular location by immunofluorescence microscopy. We found that this protein is located in nucleoid-associated clusters, that the number of foci correlates with the number of replication forks at any cell age, and that after the replication process ends the number of cells containing NrdB foci decreases.

We show that the number of NrdB foci is very similar to the number of SeqA, DnaB, and DnaX foci, both in the whole culture and in different cell cycle periods. In addition, interfoci distances between NrdB and three replication proteins are similar to the distances between two replication protein foci.

Conclusions

NrdB is present in nucleoid-associated clusters during the replication period. These clusters disappear after replication ends. The number of these clusters is closely related to the number of replication forks and the number of three replication protein clusters in any cell cycle period. Therefore we conclude that NrdB protein, and most likely RNR protein, is closely linked to the replication proteins or replisome at the replication fork. These results clearly support the replication hyperstructure model.

Natural selection and immortality

Genomes replicate while the host cells reproduce. I explore the reproduction/replication dialogue, based on a deep analysis of bacterial genomes, in relation to ageing. Making young structures from aged ones implies creating information. I revisit Information Theory, showing that the laws of physics permit de novo creation of information, provided an energy-dependent process preserving functional entities makes room for entities accumulating information. I identify explicit functions involved in the process and characterise some of their genes. I suggest that the energy source necessary to establish reproduction while replication is temporarily stopped could be the ubiquitous polyphosphates. Finally, I show that rather than maintain and repair the original individual, organisms tend to metamorphose into young ones, sometimes totally, sometimes progressively. This permits living systems to accumulate information over generations, but has the drawback, in multicellular organisms, to open the door for immortalisation, leading to cancer.