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S Mitra

Publications and source records attributed to S Mitra.

At least 307 records · Page 17Linked to original sources
PubMed1984

Autonomous parvovirus LuIII encapsidates equal amounts of plus and minus DNA strands.

Autonomous parvoviruses are thought to uniquely encapsidate single-stranded DNA of minus polarity. In contrast, the defective adeno-associated viruses separately encapsidate equal amounts of plus and minus DNA strands. We reexamined the uniqueness of minus strand encapsidation for the autonomous parvoviruses. Although we found that Kilham rat virus and H-1 virus encapsidate varying but small amounts of complementary-strand DNA, it was unexpected to find that LuIII virus encapsidated equal amounts of plus and minus DNA. The extracted LuIII DNA possessed properties of double-stranded replicative-form DNA, including insensitivity to S1 endonuclease, cleavage by restriction enzymes, and conversion to unit-length, single-stranded DNA when electrophoresed under denaturing conditions. However, the inability of this DNA to form single-stranded DNA circles when denatured and then renatured in the presence of formamide and the lack of double-stranded DNA circle formation after treatment with exonuclease III and reannealing shows a lack of sequence homology of the 3' and 5' termini of LuIII DNA, in contrast to adeno-associated virus DNA. Digestion of LuIII double-stranded DNA with EcoRI and HincII and separation of plus and minus DNA strands on composite agarose-acrylamide gels identified a heterogeneity present only in the plus DNA strand. These results suggest that strand specificity of viral DNA encapsidation is not a useful property for differentiation between the autonomous and defective parvoviruses. Furthermore, encapsidation by LuIII of equal amounts of complementary DNA strands in contrast to encapsidation of minus strands by H-1 virus, when propagated in the same host cell type, suggests that selection of strands for encapsidation is a virus-coded rather than host-controlled event.

Animals↗
PubMed1984

About Laplace transforms of the distribution of births in the initial population with unchanging net maternity rates.

"The solutions of the integral equation for the stable population by Laplace transform have been examined by shifting the origin from t=0 to other points in time. The relationship between the birth functions generated by the initial populations measured from different points of time has been found to be very simple and straight forward." The author also shows "that for a stable population, the total number of births in the next generation can be expressed as a simple function of the net reproduction rate, the intrinsic rate of growth and number of births at the beginning of the process. For the stationary population, the same relationship simplifies even further and is given in terms of the product of the number of births at the beginning of the process and the average age of motherhood."

Age Factors↗
PubMed1983

Electron microscopic comparison of the sequences of single-stranded genomes of mammalian parvoviruses by heteroduplex mapping.

The sequence homologies among the linear single-stranded genomes of several mammalian parvoviruses have been studied by electron microscopic analysis of the heteroduplexes produced by reannealing the complementary strands of their DNAs. The genomes of Kilham rat virus, H-1, minute virus of mice and LuIII, which are antigenically distinct non-defective parvoviruses, have considerable homology: about 70% of their sequences are conserved. The homologous regions map at similar locations in the left halves (from the 3' ends) of the genomes. No sequence homology, however, is observed between the DNAs of these nondefective parvoviruses and that of bovine parvovirus, another non-defective virus, or that of defective adenoassociated virus, nor between the genomes of bovine parvovirus and adenoassociated virus. This suggests that only very short, if any, homologous regions are present. From our results, we predict an evolutionary relationship among Kilham rat virus, H-1, minute virus of mice and LuIII. It is interesting to note that, although LuIII was originally isolated from a human cell line and is specific for human cells in vitro, its genome has sequences in common only with the rodent viruses Kilham rat virus, minute virus of mice and H-1, and not with the other two mammalian parvoviruses tested.

Base Sequence↗
PubMed1983

Quantitation of O6-methylguanine-DNA methyltransferase in HeLa cells.

A synthetic DNA polymer containing [8-3H]O6-methylguanine (m6G) was used as a substrate to assay the in situ demethylation of the alkylated base by an activity in HeLa cell extracts. The repair activity appears to be similar to the O6-methylguanine-DNA methyltransferase of E. coli and to be inactivated by reaction with the substrate. Extracts of a methylation-repair proficient (Mer+) cell strain, HeLa CCL2, were found to contain m6G repair activity equivalent to approx. 100 000 molecules of methyltransferase per cell, assuming that each molecule can demethylate one m6G residue. No activity could be detected in the extract of a repair deficient (Mer-) cell strain, HeLa S3, and there is no evidence of an inhibitor of repair activity in this strain.

DNA Repair↗
PubMed1983

Repair of O6-methylguanine in DNA by demethylation is lacking in Mer- human tumor cell strains.

The ability of extracts of human tumor cells to demethylate O6-methylguanine (O6-MeG) in DNA was assayed using the synthetic DNA polymer poly(dC,dG,m6dG). Cell strains proficient in repair of O6-MeG in vivo (Mer+ phenotype) contained a methyltransferase activity while repair deficient cells (Mer- phenotype) had little or no activity. Mixing extracts of different Mer- strains did not result in the appearance of the activity. Extracts of Mer- cells did not inhibit the activity in extracts of Mer+ cells. Both Mer+ and Mer- strains contained methylnitrosourea-damage-specific endonuclease activity. The data suggest that the Mer- strains are deficient in methyltransferase and that this is the fundamental reason for their hypersensitivity to the cytotoxic effects of DNA alkylation. The activity was partially purified from a Mer+ colon carcinoma cell strain. Its kinetics parallel the repair of O6-MeG in DNA in vivo and suggest that the activity is inactivated during repair of DNA.

DNA Repair↗
PubMed1983

Distribution of restriction enzyme sites in the bovine parvovirus genome and comparison to other autonomous parvoviruses.

A physical map has been constructed for bovine parvovirus (BPV) replicative form (RF) DNA synthesized in vitro. Analysis of restriction enzyme digestion products for 12 enzymes by neutral gel electrophoresis allowed the mapping of 26 cleavage sites between the 3' and 5' termini of the in vitro RF DNA. Cleavage sites for these enzymes were not detected within the 3'-terminal hairpin. HindIII, SalI, SmaI, SstII and XorII did not cleave BPV DNA, while 12 other enzymes cleaved the genome at one to four sites. Eleven additional enzymes were shown to cleave the genome at 8 to 20 sites. Comparison of the restriction site locations for BPV with those of other autonomous parvoviruses suggests that there may be conservation within the genome of certain restriction sites.

Animals↗
PubMed1983

Adaptive response of Bacillus subtilis to N-methyl-N'-nitro-N-nitrosoguanidine.

Cell extracts of Bacillus subtilis contain a methyltransferase that appears to remove the O6-methyl group from O6-methylguanine in DNA in situ. This reaction proceeds in a stoichiometric fashion, as in Escherichia coli. However, the basal level of the enzyme (approximately 240 molecules per cell) is significantly higher in B. subtilis than in E. coli. In addition, the methyltransferase level increases by an order of magnitude as a result of de novo protein synthesis after adaptive treatment with a low concentration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), as in E. coli. Concomitant with adaptation, B. subtilis cells become more resistant to both killing and mutagenesis by a challenge dose of N-methyl-N'-nitro-N-nitrosoguanidine. We present evidence to support the hypothesis that the majority of N-methyl-N'-nitro-N-nitrosoguanidine-induced mutations in B. subtilis are of the guanine-to-adenine transition type.

Adaptation, Physiological↗
PubMed1983

Generalization of the immigration and the stable population model.

Building upon an idea presented by Espenshade, Bouvier and Arthur (1982) about the eventual stationarity of a population subjected to a net reproduction rate of less than one and a constant stream of immigration, this note reports the consequences upon the birth trajectory for all possible values of the net reproduction rate. It has been shown that when the net reproduction rate is equal to one, the number of births increases linearly with time. As may be expected, the increase is exponential for values of net reproduction rate greater than one.

Emigration and Immigration↗
PubMed1983

Xeroderma pigmentosa.

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Carcinoma, Squamous Cell↗
PubMed1983

A simple model for linking life tables by survival-mortality ratios.

Patterns of variation in mortality can be studied by measuring changes in selected life table functions. A model is proposed in which the rate of change over time in the life table survivorship probability at any age has been assumed as proportional to the product of its own value and its complementary probability or the probability of dying by that age, where the proportion is the same for all ages and depends only on the time duration between successive life tables. The end result is that the logit functions of the survivorship probabilities at two points in time are linearly related with a slope of one. The projecting power of the model has been tested by using U.S. life tables for the years 1950 and 1970 as well as Coale and Demeny's regional model life tables. In the latter case, the model produced surprisingly close matches even when the expectations of life differed by as much as 20 years.

Adolescent↗
PubMed1982

Removal of O6-methylguanine from DNA by human liver fractions.

In in vitro assays using methylated DNAs as substrates, human liver fractions were shown to be able to catalyze the removal of O6-methylguanine. The amount of removal was proportional to the amount of protein added, and the loss of O6-methylguanine occurred with stoichiometric formation of guanine in the DNA and S-methylcysteine in protein. This indicates that human liver contains a protein similar to that previously found in bacteria exposed to alkylating agents. This protein acts as a transmethylase, transferring the intact methyl group from O6-methylguanine in DNA to a cysteine residue on that protein. A similar activity is present in rodent liver, but it was found that human liver was about 10 times more active in carrying out this reaction. In contrast, there was no difference between the human and rat liver extracts in catalyzing the loss of another methylation product, 7-methylguanine, from alkylated DNA. The liver is the organ most likely to be alkylated after exposure to exogenous potential alkylating agents such as dimethylnitrosamine. The present results show that human liver has a significant capacity to repair O6-methylguanine in DNA, which has been implicated as a critical product in carcinogenesis and mutagenesis.

Animals↗
PubMed1982

Proton NMR of the histidines of azurin from Alcaligenes faecalis: linkage of histidine-35 with redox kinetics.

On the basis of redox kinetic studies, Rosen and Pecht [Rosen, P. & Pecht, I. (1976) Biochemistry 15, 775-786] postulated a slowly attained (approximately equal to 0.1 sec) conformational equilibrium between two forms of reduced azurin from the bacterium Pseudomonas aeruginosa, one form being faster in electron transfer. NMR investigations have shown that at pH 7 there are two forms of reduced azurin exchanging slowly with each other, differing in the presence or absence of a proton on the imidazole side chain of histidine-35. Rosen et al. [Rosen, P., Segal, M. & Pecht, I. (1981) Eur. J. Biochem. 120, 339-344] observed that the azurin from the bacterium Alcaligenes faecalis shows no such slowly attained equilibrium between two forms. Therefore, a 1H NMR study was carried out on this azurin with emphasis on the downfield region. A resonance was found at 7.95 ppm downfield that does not move with pH, is not seen in the oxidized protein, has the same pseudocontact shift in the Co(II) derivative as the C-2 proton of histidine-35 has in the Co(II) derivative of P. aeruginosa azurin, and, in the apoprotein, exhibits a typical protonation shift downfield at pH less than 5. Therefore, this resonance is assigned to the C-2 proton of histidine-35. The crystal structure of P. aeruginosa azurin shows that at pH 7 the imidazole side chain of histidine-35 is in a crevice within the protein, where its ring is adjacent and parallel to that of histidine-47, a copper ligand. The preceding observations combined with others show that the kinetics of some redox reactions involving azurin depend on the position of histidine-35. The implication is that there is a pathway for electron transport to the copper atom involving passage through histidine-35.

Alcaligenes↗
PubMed1982

Mutagenesis of bacteriophage T7 in vitro by incorporation of O6-methylguanine during DNA synthesis.

An in vitro system in which bacteriophage T7 DNA is replicated and efficiently packaged into procapsids to form viable phage has been used to examine mutagenesis. The fidelity of replication was assayed both by measuring reversion of an amber mutation in an essential gene and by generation of temperature-sensitive mutants among the phage produced in vitro. Under standard reaction conditions, the fidelity of DNA replication is about equal to that normally found in vivo. However, when O6-methyldeoxyguanosine triphosphate is included in the reaction, O6-methylguanine is incorporated into newly synthesized DNA and the mutation frequencies increase 10- to 70-fold over the control. These experiments demonstrate in vitro mutagenesis with the T7 DNA replication-packaging system and provide more direct evidence for the premutagenic role of O6-methylguanine.

Alkylation↗
PubMed1982

Comparative physicochemical and biological properties of two strains of Kilham rat virus, a non-defective parvovirus.

Two antigenically indistinguishable strains, 171 and 308, of Kilham rat virus (KRV) have distinct host ranges and contain capsid proteins of identical size, but with different isoelectric points. The single-stranded DNA genomes of the viruses are also the same size but appear to have different secondary and tertiary structures. The genomes of the two strains have nearly identical cleavage maps of 11 restriction endonucleases, except for differences in five restriction sites in the region bracketed by 0.63 to 0.90 map units (from the 3' ends of the virus strands). However, there is a lack of extended heterology in the nucleotide sequence of the two virus genomes, as judged by electron microscopic analysis of the heteroduplex of the two virus DNAs. This suggests that very subtle differences in the sequences of the genome, and possibly of the capsid proteins, may play a role in the host specificity without affecting the antigenic similarity of KRV strains.

Animals↗