A temperature-dependent pBR322 copy number mutant resulting from a Tn5 position effect

Silencing Transcription from an Influenza Reverse Genetics Plasmid in E. coli Enhances Gene Stability

Reverse genetics (RG) systems have been instrumental for determining the molecular aspects of viral replication, pathogenesis, and for the development of therapeutics. Here, we demonstrate that genes encoding the influenza surface antigens hemagglutinin and neuraminidase have varying stability when cloned into a common RG plasmid and transformed into Escherichia coli. Using GFP as a reporter, we demonstrate that E. coli expresses the target genes in the RG plasmid at low levels. Incorporating lac operators or a transcriptional terminator into the plasmid reduced expression and stabilized the viral genes to varying degrees. Sandwiching the viral gene between two lac operators provided the largest contribution to stability and we confirmed the stabilization is Lac repressor-dependent and crucial for subsequent plasmid propagations in E. coli. Viruses rescued from the lac operator-stabilized plasmid displayed similar kinetics and titers to the original plasmid in two different viral backbones. Together, these results indicate that silencing transcription from the plasmid in E. coli helps to maintain the correct influenza gene sequence and that the lac operator addition does not impair virus production. It is envisaged that sandwiching DNA segments between lac operators can be used for reducing DNA segment instability in any plasmid that is propagated in E. coli which express the Lac repressor.

RecBCD, SbcCD and ExoI process a substrate created by convergent replisomes to complete DNA replication

The accurate completion of DNA replication on the chromosome requires RecBCD and structure specific SbcCD and ExoI nucleases. However, the substrates and mechanism by which this reaction occurs remains unknown. Here we show that these completion enzymes operate on plasmid substrates containing two replisomes, but are not required for plasmids containing one replisome. Completion on the two-replisome plasmids requires RecBCD, but does not require RecA and no broken intermediates accumulate in its absence, indicating that the completion reaction occurs normally in the absence of any double-strand breaks. Further, similar to the chromosome, we show that when the normal completion reaction is prevented, an aberrant RecA-mediated recombination process leads to amplifications that drive most of the instabilities associated with the two-replisome substrates. The observations imply that the substrate SbcCD, ExoI and RecBCD act upon in vivo is created specifically by two convergent replisomes, and demonstrate that the function of RecBCD in completing replication is independent of double-strand break repair, and likely promotes joining of the strands of the convergent replication forks.

Unique plasmids generated via pUC replicon mutagenesis in an error-prone thermophile derived from Geobacillus kaustophilus HTA426

The plasmid pGKE75-catA138T, which comprises pUC18 and the catA138T gene encoding thermostable chloramphenicol acetyltransferase with an A138T amino acid replacement (CATA138T), serves as an Escherichia coli-Geobacillus kaustophilus shuttle plasmid that confers moderate chloramphenicol resistance on G. kaustophilus HTA426. The present study examined the thermoadaptation-directed mutagenesis of pGKE75-catA138T in an error-prone thermophile, generating the mutant plasmid pGKE75(αβ)-catA138T responsible for substantial chloramphenicol resistance at 65°C. pGKE75(αβ)-catA138T contained no mutation in the catA138T gene but had two mutations in the pUC replicon, even though the replicon has no apparent role in G. kaustophilus. Biochemical characterization suggested that the efficient chloramphenicol resistance conferred by pGKE75(αβ)-catA138T is attributable to increases in intracellular CATA138T and acetyl-coenzyme A following a decrease in incomplete forms of pGKE75(αβ)-catA138T. The decrease in incomplete plasmids may be due to optimization of plasmid replication by RNA species transcribed from the mutant pUC replicon, which were actually produced in G. kaustophilus. It is noteworthy that G. kaustophilus was transformed with pGKE75(αβ)-catA138T using chloramphenicol selection at 60°C. In addition, a pUC18 derivative with the two mutations propagated in E. coli at a high copy number independently of the culture temperature and high plasmid stability. Since these properties have not been observed in known plasmids, the outcomes extend the genetic toolboxes for G. kaustophilus and E. coli.

Molecular processes in biological thermosensation

Since thermal gradients are almost everywhere, thermosensation could represent one of the oldest sensory transduction processes that evolved in organisms. There are many examples of temperature changes affecting the physiology of living cells. Almost all classes of biological macromolecules in a cell (nucleic acids, lipids, proteins) can present a target of the temperature-related stimuli. This review discusses some features of different classes of temperature-sensing molecules as well as molecular and biological processes that involve thermosensation. Biochemical, structural, and thermodynamic approaches are applied in the paper to organize the existing knowledge on molecular mechanisms of thermosensation. Special attention is paid to the fact that thermosensitive function cannot be assigned to any particular functional group or spatial structure but is rather of universal nature. For instance, the complex of thermodynamic, structural, and functional features of hemoglobin family proteins suggests their possible accessory role as “molecular thermometers”.

Plasmid-borne prokaryotic gene expression: sources of variability and quantitative system characterization

One aim of synthetic biology is to exert systematic control over cellular behavior, either for medical purposes or to "program" microorganisms. An engineering approach to the design of biological controllers demands a quantitative understanding of the dynamics of both the system to be controlled and the controllers themselves. Here we focus on a widely used method of exerting control in bacterial cells: plasmid vectors bearing gene-promoter pairs. We study two variants of the simplest such element, an unregulated promoter constitutively expressing its gene, against the varying genomic background of four Escherichia coli cell strains. Absolute protein numbers and rates of expression vary with both cell strain and plasmid type, as does the variability of expression across the population. Total variability is most strongly coupled to the cell division process, and after cell size is scaled away, plasmid copy number regulation emerges as a significant effect. We present simple models that capture the main features of the system behavior. Our results confirm that complex interactions between plasmids and their hosts can have significant effects on both expression and variability, even in deliberately simplified systems.

A family of arabinose-inducible Escherichia coli expression vectors having pBR322 copy control

The pBAD series of expression vectors have been widely used in Escherichia coli, Salmonella enterica, and related bacteria. However, a complication with pBAD24, the most popular of these plasmids, is that it does not contain the complete functional replication origin of pBR322 as was depicted in the original paper. Instead, pBAD24 has a pBR322-derived origin that lacks the rop gene that negatively regulates copy number and thus pBAD24 has an appreciably higher copy number than that of pBR322, particularly at elevated growth temperatures. A rop-containing derivative of pBAD24 (called pBAD322) having the copy number of pBR322 is reported together with derivatives of pBAD322 that encode resistance to chloramphenicol, kanamycin, tetracycline, spectinomycin/streptomycin, gentamycin, or trimethoprim in place of ampicillin.

The native Pseudomonas stutzeri strain Q chromosomal integron can capture and express cassette-associated genes

The integron-gene cassette system contributes to multiple antibiotic resistance in bacteria and is likely to be of broader evolutionary significance. However, the majority of integron diversity consists of chromosomal integrons (CIs), with mostly unknown phenotypes, which are poorly characterized. A pUC-based reporter plasmid (pUS23) was developed containing a recombination site [aadB59 base element (59-be)] upstream of promoterlessaadB[gentamicin (Gm) resistance] andgfp(green fluorescence) genes, and this construct was used to investigate the recombination and expression activities of the CI inPseudomonas stutzeristrain Q. Electroporation of pUS23 intoP. stutzeriQ gave ampicillin-resistant transformants, which yielded GmRgreen fluorescent recombinants after plating on Gm medium. Site-specific integration of pUS23 atattIwas detected by PCR in 8 % of GmRcolonies and the frequency ofattIintegration was estimated as 2·0×10−8perP. stutzeriQ(pUS23) cell. RT-PCR confirmed integron-mediated expression ofaadBin one recombinant strain (Q23-17) and a promoter (Pc) was localized to the 5′ end of theintIgene. The integrated pUS23 and flanking integron DNA were cloned from genomic DNA of strain Q23-17 and sequenced, confirming that site-specific integration of the entire reporter plasmid had occurred at theattIsite. An insertion sequence (ISPst5; IS5family) was discovered in the vector backbone of the reporter plasmid integrated atattIand also in a pUS23 derivative recovered as a plasmid inEscherichia coliJM109. This is the first demonstration that wild-type CIs can capture gene cassettes and express cassette-associated genes.

Rapid mapping of Escherichia coli::Tn5 insertion mutations by REP-Tn5 PCR

We describe a novel method to map chromosomal Escherichia coli::Tn5 insertion mutations rapidly. This method utilizes the ends of Tn5 and the E. coli REP sequence as primer binding sites for the polymerase chain reaction (PCR). The unique E. coli chromosomal sequence located between these primer binding sites is amplified by PCR and used as a probe to identify the recombinant clones from the Kohara phage ordered E. coli miniset bank that contains the Tn5 mutated loci. We used this approach to map two Tn5 insertion mutations previously identified by their effect on glycerol metabolism. The insertion mutations mapped to glpD, the aerobic sn-glycerol-3-phosphate dehydrogenase gene. Phenotypic analysis of the mutant strains revealed one with partial GlpD activity, suggesting transposon-mediated alteration of promoter activity. This mapping method should be applicable to the rapid physical mapping of any insertion mutation in the E. coli chromosome.

Characterization of the cloned BamHI restriction modification system: its nucleotide sequence, properties of the methylase, and expression in heterologous hosts

The BamHI restriction modification system was previously cloned into E. coli and maintained with an extra copy of the methylase gene on a high copy vector (Brooks et al., (1989) Nucl. Acids Res. 17, 979-997). The nucleotide sequence of a 3014 bp region containing the endonuclease (R) and methylase (M) genes has now been determined. The sequence predicts a methylase protein of 423 amino acids, Mr 49,527, and an endonuclease protein of 213 amino acids, Mr 24,570. Between the two genes is a small open reading frame capable of encoding a 102 amino acid protein, Mr 13,351. The M. BamHI enzyme has been purified from a high expression clone, its amino terminal sequence determined, and the nature of its substrate modification studied. The BamHI methylase modifies the internal C within its recognition sequence at the N4 position. Comparisons of the deduced amino acid sequence of M. BamHI have been made with those available for other DNA methylases: among them, several contain five distinct regions, 12 to 22 amino acids in length, of pronounced sequence similarity. Finally, stability and expression of the BamHI system in both E. coli and B. subtilis have been studied. The results suggest R and M expression are carefully regulated in a 'natural' host like B. subtilis.

Mutational analysis of the Escherichia coli glpFK region with Tn5 mutagenesis and the polymerase chain reaction

Transposon Tn5 mutagenesis of the Escherichia coli chromosome was used to isolate 21 independent insertion mutations conferring an altered colony color phenotype on MacConkey-glycerol plates. The polymerase chain reaction was used to map 16 of these Tn5 insertions within the glpFK region at 88 min. The most polar Tn5 insertion was shown by nucleotide sequencing to be in the proposed glpF open reading frame. The data suggest that the glpF and glpK genes are in an operon with a bent DNA segment (BENT-6) involved in transcriptional regulation of this operon.

Genetic and sequence organization of the mcrBC locus of Escherichia coli K-12

The mcrB (rglB) locus of Escherichia coli K-12 mediates sequence-specific restriction of cytosine-modified DNA. Genetic and sequence analysis shows that the locus actually comprises two genes, mcrB and mcrC. We show here that in vivo, McrC modifies the specificity of McrB restriction by expanding the range of modified sequences restricted. That is, the sequences sensitive to McrB(+)-dependent restriction can be divided into two sets: some modified sequences containing 5-methylcytosine are restricted by McrB+ cells even when McrC-, but most such sequences are restricted in vivo only by McrB+ McrC+ cells. The sequences restricted only by McrB+C+ include T-even bacteriophage containing 5-hydroxymethylcytosine (restriction of this phage is the RglB+ phenotype), some sequences containing N4-methylcytosine, and some sequences containing 5-methylcytosine. The sequence codes for two polypeptides of 54 (McrB) and 42 (McrC) kilodaltons, whereas in vitro translation yields four products, of approximately 29 and approximately 49 (McrB) and of approximately 38 and approximately 40 (McrC) kilodaltons. The McrB polypeptide sequence contains a potential GTP-binding motif, so this protein presumably binds the nucleotide cofactor. The deduced McrC polypeptide is somewhat basic and may bind to DNA, consistent with its genetic activity as a modulator of the specificity of McrB. At the nucleotide sequence level, the G+C content of mcrBC is very low for E. coli, suggesting that the genes may have been acquired recently during the evolution of the species.

Acinetobacter calcoaceticus genes involved in biosynthesis of the coenzyme pyrrolo-quinoline-quinone: nucleotide sequence and expression in Escherichia coli K-12

Synthesis of the coenzyme pyrrolo-quinoline-quinone (PQQ) from Acinetobacter calcoaceticus requires the products of at least four different genes. In this paper we present the nucleotide sequence of a 5,085-base-pair DNA fragment containing these four genes. Within the DNA fragment three reading frames are present, coding for proteins of Mr 10,800, 29,700, and 43,600 and corresponding to three of the PQQ genes. In the DNA region where the fourth PQQ gene was mapped the largest possible reading frame encodes for a polypeptide of only 24 amino acids. Still, the expression of this region is essential for the biosynthesis of PQQ. A possible role for this DNA region is discussed. Sandwiched between two PQQ genes an additional reading frame is present, coding for a protein of Mr 33,600. This gene, which is probably transcribed in the same operon as three of the PQQ genes, seems not required for PQQ synthesis. Expression of the PQQ genes in Acinetobacter lwoffi and Escherichia coli K-12 led to the synthesis of the coenzyme in these organisms.

Role of the 5' upstream sequence and tandem promoters in regulation of the rpsU-dnaG-rpoD macromolecular synthesis operon

Bal31 exonuclease deletion analysis and transposon Tn5 mutagenesis of the 5' regulatory region of the rpsU-dnaG-rpoD macromolecular synthesis operon fused to the chloramphenicol acetyltransferase gene (pGLR301) demonstrated that sequences 5' to the operon promoters were not involved in operon transcriptional regulation and that the three tandem promoters P1, P2, and P3 were functionally independent. P2 was the strongest promoter, and P3 was the weakest. P1, P2, and P3 acting in combination appeared to be stronger than the individual promoters.

Alcohol dehydrogenase gene from Alcaligenes eutrophus: subcloning, heterologous expression in Escherichia coli, sequencing, and location of Tn5 insertions

The nucleotide sequence of the gene that encodes the fermentative, multifunctional alcohol dehydrogenase (ADH) in Alcaligenes eutrophus, and of adjacent regions on a 1.8-kilobase-pair PstI fragment was determined. From the deduced amino acid sequence, a molecular weight of 38,549 was calculated for the ADH subunit. The amino acid sequence reveals homologies from 22.3 to 26.3% with zinc-containing alcohol dehydrogenases from eucaryotic organisms (Schizosaccharomyces pombe, Zea mays, mouse, horse liver, and human liver). Most of the 22 amino acid residues, which are strictly conserved in this group of ADHs (H. Jörnvall, B. Persson, and J. Jeffery, Eur. J. Biochem. 167:195-201, 1987), either were present in the A. eutrophus enzyme or had been substituted by related amino acids. The A. eutrophus adh gene was transcribed in Escherichia coli only under the control of the lac promoter, but was not expressed by its own promoter. A sequence resembling the E. coli consensus promoter DNA sequence did not contain the invariant T, but a G, in the potential -10 region. In the transposon-induced mutants HC1409 and HC1421, which form ADH constitutively, the insertions of Tn5::mob were localized 56 and 66 base pairs, respectively, upstream of the presumptive translation initiation codon. In contrast to the promoter, the A. eutrophus ribosome-binding site with a GGAG Shine-Dalgarno sequence 6 base pairs upstream of the translation initiation codon was accepted by the E. coli translation apparatus. A stable hairpin structure, which may provide a transcription termination signal, is predicted to occur in the mRNA, with its starting point 21 base pairs downstream from the translation termination codon.

Expression of a cloned Bacillus thuringiensis crystal protein gene in Escherichia coli

The expression in Escherichia coli of a cloned crystal protein gene from Bacillus thuringiensis was investigated through the use of fusions of the crystal protein gene promoter to beta-galactosidase and catechol oxidase genes. Analysis of deletion and insertion derivatives of the crystal protein gene promoter showed that a region of B. thuringiensis DNA located between 87 and 258 base pairs upstream from the transcription initiation site caused reduced transcription from this promoter. Insertion of Tn5 145 base pairs upstream from the transcription initiation site resulted in overproduction of the crystal protein. S1 nuclease mapping experiments failed to detect transcription from an outwardly directed promoter in Tn5, indicating that the overproduction resulted from the disruption or repositioning of the transcription-suppressing region.