Conjugation in Escherichia coli

Two DsbA Proteins Are Important for Vibrio parahaemolyticus Pathogenesis

Bacterial pathogens maintain disulfide bonds for protein stability and functions that are required for pathogenesis. Vibrio parahaemolyticus is a Gram-negative pathogen that causes food-borne gastroenteritis and is also an important opportunistic pathogen of aquatic animals. Two genes encoding the disulfide bond formation protein A, DsbA, are predicted to be encoded in the V. parahaemolyticus genome. DsbA plays an important role in Vibrio cholerae virulence but its role in V. parahaemolyticus is largely unknown. In this study, the activities and functions of the two V. parahaemolyticus DsbA proteins were characterized. The DsbAs affected virulence factor expression at the post-translational level. The protein levels of adhesion factor VpadF (VP1767) and the thermostable direct hemolysin (TDH) were significantly reduced in the dsbA deletion mutants. V. parahaemolyticus lacking dsbA also showed reduced attachment to Caco-2 cells, decreased β-hemolytic activity, and less toxicity to both zebrafish and HeLa cells. Our findings demonstrate that DsbAs contribute to V. parahaemolyticus pathogenesis.

Conjugation in Escherichia coli: A laboratory exercise

Bacterial conjugation is a genetic transfer that involves cell-to-cell between donor and recipient cells. With the current method used to teach students in genetic courses at the undergraduate level, the transconjugants are identified using bacterial physiology and/or antibiotic resistance. Using physiology, however, is difficult for both first-year undergraduates and special science students at the high school levels, who do not have the basic knowledge. We have developed a laboratory exercise that comprises a simple and rapid technique for transferring bacterial DNA by conjugation and examining the transconjuants using only antibiotic resistance on agar. The identity of the right transconjuants is confirmed by agarose gel electrophoresis. This exercise is designed to help students understand how horizontal gene transfer occurs in bacteria by conjugation using Escherichia coli as a hands-on learning model. Students should be able to draw concept maps of three DNA transfer methods on their own after carrying out the experiment and getting some additional information.

Improved oligonucleotide site-directed mutagenesis using M13 vectors

An improved method is described for the construction of mutations in M13 vectors using synthetic oligonucleotides. The DNA is first cloned into a novel M13 vector (based upon M13mp18 or M13mp19), which carries a genetic marker that can be selected against, such as an EcoK or EcoB site, or an amber mutation in an essential phage gene. In this "coupled priming" technique, one primer is used to construct the silent mutation of interest, and a second primer is used to eliminate the selectable marker on the minus strand. After primer extension and ligation, the heteroduplex DNA is transfected into a strain of E. coli which is repair deficient and selects against the plus strand marker. Over 50 mutants have been constructed with this approach, and the yields can be excellent (up to 70%). For the stepwise construction of mutations using separate rounds of mutagenesis, the EcoK and EcoB markers offer a particular advantage over the amber marker. They permit selection in each round, as it is possible to cycle between the two markers. However for construction of multiple mutations over a short region, long synthetic oligonucleotides with multiple mismatches to the template can offer an alternative strategy.

Prophage induction and cell division in E. coli. III. Mutations sfiA and sfiB restore division in tif and lon strains and permit the expression of mutator properties of tif

In E. coli K12, cell filamentation promoted by tif is enhanced by the lon mutation; in contrast, prophage induction and repair of UV-irradiated phage lambda, also promoted by tif, are not affected by lon. From a tif lon double mutant, "revertants" having recovered the ability to divide at 41 degrees were isolated, among which most (95%) had also lost their Lon filamentous phenotype after ultraviolet (UV) irradiation. From these 95% of revertants: (1) 94% are suppressed for the whole Tif phenotype, by additional mutations that render them deficient in DNA repair, as judged from their high UV sensitivity; some have been characterized as recA mutants. (2) 1% have recovered a control on cell division at 41 degrees or after UV irradiation by means of secondary mutations altering neither the other phenotypic properties of tif and lon, nor the repair and recombination ability of the cells: in particular, this class of "revertants" remains thermoinducible upon lysogenisation; the mutations which specifically suppress filamentation have been mapped at two loci, sfiA and sfiB, cotransducible respectively with pyrD and leu. In the remaining 5% of revertants that still exhibit an UV-induced filamentous growth, 3% can be tentatively classified as true tif+ revertants; 2% behave as tif thermodependent revertants, showing suppression of the Tif (and Lon) phenotype only at 41 degrees: 2recAts have been identified in this class. Non-lysogenic tif lon sfi and tif sfi strains remain viable during prolonged growth at 41 degrees. Under these conditions, tif expresses mutator properties, which can be conveniently analyzed in this sfi background. The action of lif, lon and sfi mutations is tentatively interpreted on the basis of a negative control of cell division specifically associated with DNA repair.

The effect of sul on permissiveness of Plkc in strain B of Escherichia coli

Strain B/r (Witkin) of Escherichia coli unlike strain B is permissive for the growth of Plkc. All 485 radiation resistant (sul) mutants of strain B, selected from among survivors of ultraviolet radiation or methylmethane sulfonate were permissive for Plkc. Permissiveness for Plkc followed the sul marker when transferred into strain B by sexual recombination or Pl transduction. Though sul is known to suppress some of the properties of lon, the gene associated with UV sensitivity and filamentation in strain B, lon, itself was found not to be involved in permissiveness for Plkc. Thus lon mutants of strain K12 were and lon+derivatives of strain B were not necessarily permissive for Plkc. A gene, not lon+, could be transferred from K12 to strain B by sexual recombination which conferred permissiveness for Plkc on the recipients. This gene maps between leu and proA. It's relationship to sul+is discussed.

Genetic analysis of the leucine region in Escherichia coli B-r: gene-enzyme assignments

Genetic mapping by transduction and conjugation using F(-) and F' strains carrying either point mutations in the l-arabinose or leucine regions or ara-leu fusion-deletion mutations has resulted in a detailed genetic map of the arabinose-leucine region of Escherichia coli B/r. These studies have identified four genes in the leucine region having the same order as found in Salmonella typhimurium: ara... leuDCBA.

Transport-limited growth rates in a mutant of Escherichia coli

Mutants of Escherichia coli B/r have been selected which require increased nutrient concentrations for half-maximal growth rate. This half-saturation constant (K(m)) for growth on glucose is 10(-6) and 7 x 10(-4)m for the wild type (CP 366) and the mutant (CP 367), respectively. Similarly, the K(m) is increased for growth on many other carbohydrates (20- to 500-fold), for the anions PO(4) (3-) and SO(4) (2-) (ca. 100-fold), and for the uptake of several amino acids (20- to 50-fold). At sufficiently high concentrations of the nutrients, mutant and wild type grow equally fast. The yield in terms of cell mass per milligram of substrate is unaffected by the mutation. The phenotype of the parent is reestablished in what appears to be the reversion of a single mutation (kmt) which maps between strA and metB. The pleiotropic decrease of the affinities for transport of the various nutrients seems to be the result of a modification of the cell envelope which weakens the attachment of the various specific binding proteins to the periplasmic membrane. Since the mutant K(m) values are increased considerably, high cell densities can be reached in batch cultures at growth-rate-limiting substrate concentrations (10(7) to 10(8) cells/ml). This allows chemical analysis of the cell composition; the application of the mutant to studies of bacterial physiology as function of growth rate is discussed.

Genetic and molecular characteristics of X-ray-sensitive mutants of Escherichia coli defective in repair synthesis

Alleles responsible for X-ray-sensitive characteristics of three mutants of Escherichia coli B, which were also sensitive to ultraviolet (UV) irradiation, were mapped near metE locus, and named res-1, res-2, and res-3. All the res(-) mutants showed no host cell reactivability (Hcr(-)) for transducing deoxyribonucleic acid (DNA) of P1 phage irradiated by UV but they were Hcr(+) for infective DNA of P1 phage. Furthermore, they showed no detectable activity of DNA polymerase. Characteristics of allele res-1 were studied in detail. The mutant res-1 uvr(+) showed an extensive degradation of DNA after UV irradiation. Double mutants carrying res-1 uvrA(-), res-1 uvrB(-), and res-1 uvrC(-) showed no marked increase in UV sensitivity beyond that of the uvr(-) single mutants and only negligible UV-induced DNA degradation. The uvr(-) mutations showed no such suppressive effect on DNA degradation induced by X rays in these double mutants. It is concluded that res(-) mutants are defective in the second step (repair synthesis) of the excision repair process and that DNA polymerase is partly responsible for the assumed resynthesis step.

Genetic control of the secondary modification of deoxyribonucleic acid in Escherichia coli

The wild-type restriction and modification alleles of Escherichia coli K-12 and B were found to have no measurable effect on the patterns of methylated bases in the deoxyribonucleic acid (DNA) of these strains. The genetic region controlling the methylation of cytosine in E. coli K-12 was mapped close to his, and the presence or absence of this gene in E. coli B or E. coli K had no effect on the restriction and modification properties of these strains. Thus, only a few of the methylated bases in the DNA of these strains are involved in host modification, and the biological role of the remainder remains obscure.

Genetic analysis of a large-cell, radiation-resistant strain of Escherichia coli

A genetic analysis of Escherichia coli P6, a large-cell, radiation-resistant strain of E. coli, established that it originated as the result of a mutational event. The gene responsible for the complex P6 phenotype was located at 61 +/- 0.5 min on the E. coli linkage map. The close resemblance of conjugal and transductional recombinants to one or the other parent without indication of an intermediate class suggests that only a single gene may be involved.

Origin and direction of replication of the chromosome of E. coli B-r

The origin and direction of replication of the E. coli B/r chromosome has been determined by comparing gene frequencies in P1-transducing lysates prepared on cultures growing at different rates. The gene frequencies found are consistent with the idea that replication of the chromosome is dichotomous in rapidly growing B/r. The origin was found to be between 40 and 55 min on the E. coli genetic map with replication proceeding in a clockwise direction. Markers near the origin behaved anomalously.

Chromosomal Location of a Gene Involved in Potassium Ion Uptake in Escherichia coli B

A chromosomal lesion responsible for defective potassium ion uptake in Escherichia coli B has been mapped by use of standard interrupted-mating crosses. The mutation, kac-1 , is in strain RD-2, which is deficient in K + intake, exchanges cell K + for extracellular isotope at a reduced rate, and has an abnormality of phosphorus metabolism associated with its potassium deficiency. This report places kac-1 at about 4 min clockwise from pro , close to gal . The locus of kac-1 is distinctly different from the potassium retention mutant in strain B-207, the only other potassium accumulation mutant mapped in E. coli so far. In this study, two other potassium accumulation mutations, kac-2 and kac-3 , whose particular type of accumulation defects have not yet been determined, were mapped. These mutations are in the same region of the chromosome as kac-1 .

Arabinose-leucine deletion mutants of Escherichia coli B-r

The control of ara gene expression was studied in mutants of Escherichia coli B/r containing deletions which fused the l-arabinose gene complex with the leucine operon (the normal gene order being araDABIOC...leuDCBAO). Complementation experiments with stable merodiploids showed that expression of ara genes cis to araC-leu deletions was controlled by the trans-acting product of the araC gene. Expression of ara genes cis to araB-leu deletions was under leucine control. These studies confirm the existence of a region between genes araC and araB essential for normal activator controlled expression of the ara structural genes. One deletion was characterized as an araO-leu deletion. Its effect on ara gene expression was unique in that ara genes were susceptible to potential regulation by both l-arabinose and leucine. These experiments suggest that two different species of messenger ribonucleic acid (mRNA) may be produced for the ara-leu region as a result of this deletion. One, under l-arabinose-activator control, is initiated in the l-arabinose region; the other, under leucine control, is initiated in the leucine region. The latter indicates that araI can be transcribed. Whether araI is transcribed in the former instance (mRNA made under activator control) remains to be established.

Origin and sequence of chromosome replication in Escherichia coli B-r

The initial rates of induced synthesis of tryptophanase, beta-galactosidase, and d-serine deaminase were measured in relation to the chromosome replication cycle of Escherichia coli B/r. Exponentially growing cultures were exposed briefly to (14)C-thymidine or the appropriate inducers (or both), and the amount of label or enzyme (or both) in cells of different ages was found by measuring these quantities in their progeny. The rates of induced synthesis of the three enzymes increased abruptly at about 4, 20, and 34 min, respectively, after the start of a round of replication lasting 40 min. By matching this sequence to the ind, lac, and Dsd loci on the genetic map of E. coli K-12, it was estimated that replication began at about 8 o'clock (60 min) and proceeded clockwise. In rapidly growing cells, the sequence during the division cycle was consistent with the concept that rounds of replication overlapped.

Loci of radiation sensitivity in Bs strains of Escherichia coli

The genes responsible for u.v. sensitivity in ten sensitive mutants ofE. colistrain B (Bs strains of R. Hill) have been mapped by transduction. Theuvrgenes of all the mutants able to reactivate u.v.-irradiated phage (HCR+), including Bs2, 4, 5, 6, 7, 9 and 10, were linked, 50–95%, withmalB. The gene of Bs12 (HCR) was also linked tomalBas isuvr A. The gene of Bs8 (HCR) was linked togal, that of Bs3 (HCR) tohis. Transduction ofmal+from a strain withuvr A, or amal+derivative of Bs12, to Bs2, 4, 5, 6, 7, 9 or 10, yielded about 30% u.v.-resistant transductants. Amal+transduction with Bs2mal+as donor and other Bs strains as recipients yielded < 0·1% u.v.-resistant transductants. ThemalB-linkeduvrgenes of all mutants (except Bs3 and Bs8) were transducible withmetA. The quasi-reciprocal crosses and three-point tests suggested the order of markers asmetA malB uvr(HCR+)uvr(HCR). The sensitivity gene of Bs9 was exceptional in that it appeared to lie betweenmetAandmalB. The sensitivity gene of Bs11 could not be mapped because transductants to that strain were not possible, nor did it act as a recipient in sexual recombinations.

Host-controlled restriction of T-even bacteriophages: relation of endonuclease I and T-even-induced nucleases to restriction

Restriction of nonglucosylated T2 phage (T(*)2) as a function of bacterial growth state was the same for endonuclease I-containing and endonuclease I-deficient strains of Escherichia coli B. Furthermore, E. coli strains with various levels of restriction for T2 had comparable endonuclease I activities. It was also found that a T4 mutant temperature-sensitive for gene 46 and 47 functions was fully restricted at 42 C. It therefore appears that neither endonuclease I nor the phage-induced nucleases whose activities are blocked by mutations in genes 46 and 47 catalyze the initial event in restriction of nonglucosylated T-even phages.

Mutants of Escherichia coli defective in ribonucleoside and deoxyribonucleoside catabolism

From Escherichia coli B, mutants were prepared that lacked the enzymes adenosine deaminase, cytidine deaminase, and purine nucleoside phosphorylase. In each case, the mutant lacked enzyme activity for both ribonucleoside and deoxyribonucleoside. Mutants lacking purine nucleoside phosphorylase lost the capacity to cleave the nucleosides of adenine, guanine, and hypoxanthine.