[Photosensitive metabolic network in the cells of bacteria Escherichia coli: spectral selectivity and reparative functions of globin photoreceptors]

The influence of He-Ne laser radiation (632.8), incoherent monochromatic red light (601-685 nm), as well as pulsed radiation semiconductor laser (660 nm) on the growth of the culture of wild type or uvrA strain of E. coli WP2 and on the DNA transformation of plasmid pDick (Amp, Km) in E. coli NM522 was investigated. After light signal, photoenhancement of the quantity of cells in culture retains during several generations, while the enhancement of the transformation efficiency was observed only for the kanamycin marker. This makes it possible to suppose that metabolic signaling network acts as a red light receptor. The observed spectral selectivity suggests that flavohemoglobin fatty acid peroxidase works as a photosensitive component in the reparative network. It is proposed that the enhanced effectiveness of light in the uvrA strain results from photoactivation ofglobin EcDOS phosphodiesterase as an O2 provider for AlkB demethylase.

[Effects of UV-induced DNA damage on vector ligation and transformation into bacterial cells]

OBJECTIVE

To study the effects of UV irradiation on DNA ligation and transformation efficiency of the expression vector into competent bacterial cells.

METHODS

The expression vector was digested with the restriction enzyme SfiI, and the purified target DNA fragments were exposed to UV light at different wavelengths. Ligation and transformation experiments with the exposed fragments were carried out and the colony number and transformation efficiency were assessed.

RESULTS

The transformation efficiency of the DNA with a 5-min exposure to 302 nm UV was 60 colonies per nanogram of the DNA, as compared with 20400 for the DNA exposed to 365 nm UV. The time course experiment showed that prolonged DNA exposure to 365 nm UV light was associated with lowered transformation efficiency. DNA exposure for 30 min caused a reduction of the transformation efficiency to lower than 50% compared to that of DNA without UV exposure. But with a 15 min exposure, the DNA maintained a transformation efficiency more than 70%, which was sufficient for most molecular biology experiments.

CONCLUSION

In construction of the expression vector, it is advisable to prevent the target DNA from UV exposure. When UV exposure is essential, we suggest that 365 nm UV be used and the exposure time controlled within 15 min.

Analysis of comparative efficiencies of different transformation methods of E. coli using two common plasmid vectors

The efficiencies of different transformation methods of E. coli DH5Qalpha train, induced by several cations like Mg2+, Mn2+ Rb+ and especially Ca2+, with or without polyethylene glycol (PEG) and dimethyl sulfoxide (DMSO) were compared using the two commonly used plasmid vectors pCAMBIA1201 and pBI121. The widely used calcium chloride (CaCl2) method appeared to be the most efficient procedure, while rubidium chloride (RbCl) method was the least effective. The improvements in the classical CaCl2 method were found to further augment the transformation efficiency (TR)E for both the vectors like repeated alternate cycles of heat shock, followed by immediate cold, at least up to the third cycle; replacement of the heat shock step by a single microwave pulse and even more by double microwave treatment and administration of combined heat shock-microwave treatments. The pre-treatment of CaCl2-competent cells with 5% (v/v) ethanol, accompanied by single heat shock also triggered the (TR)E, which was further enhanced, when combined heat shock-microwave was applied. The minor alterations or improved approaches in CaCl2 method suggested in the present study may thus find use in more efficient E. coli transformation.

Bradyrhizobium japonicum mutants with enhanced sensitivity to genistein resulting in altered nod gene regulation

Bradyrhizobium japonicum mutants with altered nod gene induction characteristics were isolated by screening mutants for genistein-independent nod gene expression. Plasmid pZB32, carrying a nodY::lacZ transcriptional gene fusion, was introduced into B. japonicum cells that had been subjected to UV mutagenesis. Ten independent transformants producing a blue color on plates containing 5bromo-4chloro-3indolyl-beta-D-galactopyranoside but lacking genistein, indicative of constitutive expression of the nodY::lacZ reporter gene, were isolated. Beta-galactosidase activity assays revealed that while all of the 10 strains were sensitive to low concentrations of genistein, none exhibited truly constitutive nodY::lacZ expression in liquid culture. Soybean plants inoculated with three of the mutants were chlorotic and stunted, with shoot dry weights close to those of the uninoculated plants, indicating the absence of nitrogen fixation. Differences in the kinetics of nodY::lacZ expression and lipochitin oligosaccharide Nod signal production suggested that the strains carried different mutations. Some of these strains may be useful in mitigating the low root zone temperature-associated delay in soybean nodulation at the northern extent of soybean cultivation.

Mechanism by which gamma irradiation increases the sensitivity of Salmonella typhimurium ATCC 14028 to heat

Effects of irradiation and heating on survival of Salmonella typhimurium ATCC 14028 were examined by measuring DNA damage and the integrity of the cytoplasmic membrane. S. typhimurium cells fell into two distinct groups following heating: (i) heat-sensitive cells, which were rapidly inactivated at 65 degrees C and (ii) heat-resistant cells, which were only slowly inactivated at 65 degrees C. Radiation sensitivity of S. typhimurium was greater in the presence of air than in the presence of N2 gas (radiation doses required to inactivate 90% of the cells, 0.394 +/- 0.029 in air and 0.561 +/- 0.035 in N2). Recovery of the covalently closed circular form of plasmid pBR322 from S. typhimurium transformants (Ampr Tetr) was decreased by irradiation but not by heating. Heating prior to irradiation significantly decreased the recovery of plasmid DNA without affecting survival of S. typhimurium. Transformability of the recovered plasmid pBR322 was affected by neither irradiation nor heating, and mutation of antibiotic resistance genes was not detected in S. typhimurium. Heating, but not irradiation, caused destabilization of the cytoplasmic membrane, allowing penetration of hydrophobic dye. These results suggest that lethality of heating followed by irradiation for S. typhimurium was additive, reflecting irradiation-induced DNA damage and heat-induced membrane destabilization. When irradiation preceded heating in the absence of air, more cells were inactivated than was expected, because of heat-inactivating radiation-damaged DNA.

[Lack of foto reactivation of the Haemophilus influenzae transforming DNA mutated with near ultraviolet light (325-400nm)]

Three extracts from Saccharomyces cerevisiae were obtained by salting out with ammonium sulfate, these were I-G, EFRL-II-G and III-G. Fraction EFRL-II-G showed the highest photoreactivating activity on DNA str2000 irradiated with far UV light. However, the same fraction did not reactivate DNA str2000 previously inactivated by near UV irradiation. We think that the inactivation by near-UV was not due to photochemically-formed pyrimidine dimers. Decrease in spontaneous mutation frequency of cells transformed with DNA str2000 irradiated with near-UV light, was the same with the DNA treated with active or heat inactivated EFRL-II-G, therefore we may conclude that DNA lesions responsible for the effect are difference to pyrimidine dimers.

Characterization of transformation-deficient mutants of Acinetobacter calcoaceticus

Three Acinetobacter calcoaceticus transformation-deficient mutants, obtained by insertional mutagenesis with the nptll gene, have been characterized physiologically. One mutant (AAC211) was found to be completely transformation deficient, while two others, AAC213 and AAC214, were severely impaired in transformation efficiency (100-1000 times lower than the wild type). The latter applied to both chromosomal as well as plasmid DNA. Analysis of the chromosomal DNA fragments flanking the nptll gene in the mutants showed that mutants AAC213 and AAC214 had an insertion of the nptll gene in the same chromosomal region, but that they were the result of two independent mutational events, whereas the insertion in mutant AAC211 was at a different position. None of the three mutants showed phenotypic or genotypic characteristics typical of a RecA-deficient strain.

Cloning and structural characterization of the mcrA locus of Escherichia coli

Escherichia coli has DNA restriction systems which are able to recognize and attack modified cytosine residues in the DNA of incoming bacteriophages and plasmids. The locus for the McrA/RglA system of modified cytosine restriction was located near the pin gene of the defective element, e14. Hence, loss of the e14 element through abortive induction after UV irradiation caused a permanent loss of McrA restriction activity. e14 DNA encoding McrA restriction was cloned and sequenced to reveal a single open reading frame of 831 bp with a predicted gene product of 31 kDa. Clones expressing the complete open reading frame conferred both McrA and RglA phenotypes; however, a deletion derivative was found which complemented RglA restriction against nonglucosylated T6gt phage but did not complement for McrA restriction of methylated plasmid DNA. Possible explanations for this activity and a comparison with the different organization of the McrB/RglB restriction system are discussed.

Genetic exchange of transposon and integrative plasmid markers in Mycoplasma pulmonis

Matings of genetically marked derivatives of Mycoplasma pulmonis resulted in the exchange of chromosomal DNA and the appearance of doubly marked transconjugants. Transposons Tn916 and Tn4001, and a series of integrative plasmids derived from their cloned antibiotic resistance genes, were used to construct antibiotic-resistant mycoplasmal derivatives to examine this phenomenon at the molecular level. Genetic exchange occurred on agar surfaces at frequencies ranging from 3.3 X 10(-4) to 6.4 X 10(-8) transconjugants per CFU. Examination of chromosomal DNA from transconjugants by hybridization revealed that the transposons or integrated plasmids were in the same chromosomal locations as in the parental strains, indicating that exchange involved the transfer of chromosomal DNA and homologous recombination. Transfer was not affected by DNase, polyethylene glycol, EDTA, or calcium chloride but was affected by treatment of either parent with trypsin. Mixing of mating strains before plating had no effect on mating frequencies, but mating did occur in liquid media. The ability to exchange chromosomal markers was limited to selected strains of M. pulmonis; mating did not occur with Acholeplasma laidlawii or M. gallisepticum. Heat and UV inactivation studies revealed that nonviable cells could act as donors in matings. The evidence presented supports a conjugationlike mechanism involving specific trypsin-sensitive membrane components.

DNA repair and the evolution of transformation in Bacillus subtilis. II. Role of inducible repair

In Bacillus subtilis, DNA repair and recombination are intimately associated with competence, the physiological state in which the bacterium can bind, take up and recombine exogenous DNA. Previously, we have shown that the homologous DNA transformation rate (ratio of transformants to total cells) increases with increasing UV dosage if cells are transformed after exposure to UV radiation (UV-DNA), whereas the transformation rate decreases if cells are transformed before exposure to UV (DNA-UV). In this report, by using different DNA repair-deficient mutants, we show that the greater increase in transformation rate in UV-DNA experiments than in DNA-UV experiments does not depend upon excision repair or inducible SOS-like repair, although certain quantitative aspects of the response do depend upon these repair systems. We also show that there is no increase in the transformation rate in a UV-DNA experiment when repair and recombination proficient cells are transformed with nonhomologous plasmid DNA, although the results in a DNA-UV experiment are essentially unchanged by using plasmid DNA. We have used din operon fusions as a sensitive means of assaying for the expression of genes under the control of the SOS-like regulon in both competent and noncompetent cell subpopulations as a consequence of competence development and our subsequent experimental treatments. Results indicate that the SOS-like system is induced in both competent and noncompetent subpopulations in our treatments and so should not be a major factor in the differential response in transformation rate observed in UV-DNA and DNA-UV treatments. These results provide further support to the hypothesis that the evolutionary function of competence is to bring DNA into the cell for use as template in the repair of DNA damage.

[Mutagenic effects of gamma-rays on plasmid DNA in Escherichia coli]

A purified and dried DNA of plasmid pKO482 (galK+) is 10 times more resistant to the inactivating action of 60Co-gamma-rays than that of lambda phage. gamma-Irradiation of the plasmid DNA induces forward mutations of galK, the frequency of which increases linearly with the dose. The efficiency of the mutagenic action of gamma-rays on the plasmid galK locus is 10(-12) per 1 rad and per 1 base pair. The mutagenic effect of gamma-radiation but slightly depends upon bacterial recA+ gene and upon the SOS-repair system induced by UV-irradiation of the recipient cells. It is assumed that the premutational lesions induced in the purified DNA by the direct effect of gamma-radiation are fixed into mutations by misreplication.

[Induced mutagenesis of plasmid and chromosomal genes inserted into plasmid DNA. I. The mutagenic action of radiation]

This paper describes the results of treating plasmid DNA in vitro with mutagens, to obtain mutations both in plasmid genes and chromosomal genes comprised within the plasmid, thus avoiding disorganization characteristic of in vivo mutagenesis. The model system is represented by DNA of RSF2124 responsible for colicine E1 synthesis and resistance to ampicillin. Col- mutants were looked for after exposure to UV- and gamma-irradiation. The lethal effect was estimated as inactivation of the ampicillin resistance marker. After reisolation from mutant transformant of the plasmid DNA, the novel character and resistance to ampicillin proved to retain in the course of subsequent transformations and passages of transformed colonies, suggesting the mutational nature of the changes. Exposure of RSF2124 to short-wave UV-irradiation (lambda = 254 nm) produced a pronounced mutagenic effect: the relative quantity of Col- mutants under optimal conditions of mutagenesis increased about 10 times. In the case of W-reactivation (additional UV-irradiation of C600 wild type cells) of lethal lesions, a 95% reliable increase in mutagenic effect was observed. Significant enhancement of mutagenesis (about 4-fold) was detected when only recipient cells were exposed to low doses of UV (the so-called indirect UV mutagenesis). Thus, with regard to W- and indirect UV mutagenesis, the plasmid DNA behaves like DNA of temperate phages which suggests their evolutionary relationship. Treatment of plasmid DNA with acridine orange prior to UV, only protected from lethal lesions. Gamma-irradiation (60Co) at the dose producing 100-fold inactivation, increased the yield of Col- mutants by one order of magnitude. The presence of RSF2124 plasmid in a cell does not affect its UV sensitivity.

Isolation and characterization of Bacillus subtilis mutants altered in competence

We isolated and characterized four Bacillus subtilis competence-deficient mutants. The mutants were obtained by nitrosoguanidine mutagenesis and by screening for mutants unable to be transformed both on solid and in liquid medium. Most of the mutants obtained in this way were tested for their sensitivity to the DNA-damaging agents methyl methanesulfonate, mitomycin C, and UV light. Among the mutants which did not show an increased sensitivity to these agents, four were chosen for further characterization. Data were obtained which indicate that the mutants are reduced in chromosomal and plasmid transformation and in transfection, whereas they are not altered in transduction and in protoplast transformation. Transformation experiments carried out by mixing a culture of a mutant with a culture of a wild-type strain gave some complementation for competence with one of the strains. The mutants were also characterized for their capacity to bind, take up, and break down transforming DNA; furthermore, the four competence mutations were mapped, and the results indicate that they belong to four different genes.

[In vitro repair of gamma-irradiated transforming Bacillus subtilis DNA by extracts of blue-green algae]

A cell-free extract from blue-green alga Anacystis nidulans contains enzymes which repair in vitro the transforming activity of gamma-irradiated Bacillus subtilis DNA. The level of restoration of the transforming activity depends on the protein concentration in the reaction mixture, the duration of incubation and on the dose of irradiation. The repair of gamma-induced lesions is most efficient in the presence of magnesium ions, NAD and ATP. The present data indicate that the repair of transforming DNA is performed with the participation of DNA polymerase and polynucleotide ligase which function in the cell-free extract of algae.

W-mutagenesis in competent cells of Bacillus subtilis

The relative yield (Nm/N) of fluorescent mutants Ind- after the transformation of Bacillus subtilis cells by means of UV-irradiated DNA is much higher in an uvr- recipient than in an uvr+ strain, when compared at equal fluence, but practically identical at equal survival. Ind- mutations are induced by UV-irradiation of separated single strands of transforming DNA. The H-strand is much more sensitive to the mutagenic action of UV light. Preliminary irradiation of competent recipient cells by moderate UV fluences increases the survival of UV- or gamma-irradiated transforming DNA (W-reactivation) and the frequency of Ind- mutations (W-mutagenesis). During transfection of B. subtilis cells by UV-irradiated prophage DNA isolated from lysogenic cells B. subtilis (phi 105 c+) c-mutants of the phage are obtained in high yield only in conditions of W-mutagenesis, i.e. in UV-irradiated recipient cells. These data show that there is no substantial spontaneous induction of error-prone SOS-repair system in the competent cells of B. subtilis.

Radiation sensitivity of transforming DNA

Biologically active DNA isolated from Bacillus subtilis was exposed in vitro to X-rays at a concentration of 10 microgram/ml in 29 mM phosphate buffer. Radiation-induced damage to the DNA was quantitatively determined by measuring the decrease in its transforming activity (try2 locus) using B. subtilis 168M (try-) as recipient. In O2, which removes .H and eaq-, the radiation sensitivity of the DNA is less than that in N2-saturated water. In N2O, which has been shown to increase yields of .OH in irardiated aqueous solutions, the radiation sensitivity of Transforming DNA is twice that observed in O2 and 1.5 times that in N2. Addition of 5 X 10(-2) M ethanol or 1.7 X 10(-1) M t-butanol, both .OH scavengers, causes large (about tenfold) reduction in the radiation sensitivity in all three saturating gases. These results suggest the importance of the .OH radical in the loss of biological activity of DNA.