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M S Fox

Publications and source records attributed to M S Fox.

At least 37 records · Page 2Linked to original sources
PubMed1987

Fine structure recombinational analysis of cloned genes using yeast transformation.

We describe a general method for analyzing the genetic fine structure of plasmid-borne genes in yeast. Previously we had reported that a linearized plasmid is efficiently rescued by recombination with a homologous restriction fragment when these are co-introduced by DNA-mediated transformation of yeast. Here, we show that a mutation can be localized to a small DNA interval when members of a deletion series of wild-type restriction fragments are used in the rescue of a linearized mutant plasmid. The resolution of this method is to at least 30 base pairs and is limited by the loss of a wild-type marker with proximity to a free DNA end. As a means for establishing the nonidentity of two mutations, we determined the resolution of two-point crosses with a mutant linearized plasmid and a mutant homologous restriction fragment. Recombination between mutations separated by as little as 100 base pairs was detected. Moreover, the results indicate that exchange within a marked interval results primarily from one of two single crossovers that repair the linearized plasmid. These approaches to mapping the genetic fine structure of plasmids should join existing methods in a robust approach to the mutational analysis of gene structure in yeast.

Chromosome Deletion↗
PubMed1987

Some features of base pair mismatch and heterology repair in Escherichia coli.

We have used artificially constructed heteroallelic heteroduplex molecules of bacteriophage lambda DNA to transfect Escherichia coli, and E. coli mutants deficient in various functions involved in the adenine methylation-directed mismatch repair system, MutL, MutS, MutH, and UvrD (MutU). Analysis of the allele content of single infective centers shows that this repair system often acts on several mismatches, separated by as many as 2000 bp, on one of the strands of a heteroduplex molecule. When the methyl-directed mismatch repair system is disabled by mutH or uvrD mutations, localized mismatch repair becomes prominent. This prominent localized repair that can result in separation of very closely linked markers requires the functions MutL and MutS, is independent of adenine methylation, and appears to reflect another mechanism of mismatch repair. Heterology-containing heteroduplex molecules with a deletion in one strand often escape processing. However, when the heterology includes the stem and loop structure of a transposon, Tn10, the transposon is lost.

Bacteriophage lambda↗
PubMed1986

A genetic analysis of primary products of bacteriophage lambda recombination.

Primary products of bacteriophage lambda recombination that display heterozygosity as a consequence of the presence of regions of heteroduplex DNA are rare in standard lambda crosses. Phage manifesting heterozygosity at a given allele are evident when recombinants, emerging from a cross, are selected for an exchange in a neighboring interval. We show that the abundance of such heterozygotes can be increased 10- to 20-fold by selection on an E. coli indicator that is defective in methyl-directed mismatch repair (mutL). Thus, the activity of the methyl-directed mismatch repair system is, at least in part, responsible for the low frequency of detectably heterozygous phage emerging from a standard cross. In a mutL indicator, many primary products of recombination are replicated without the intervention of mismatch repair.--The products of a six-factor phage cross have been plated on a mutL indicator allowing visual detection of those phage products heterozygous for one of the allelic pairs, cI. By genetic analysis, we show that the heteroduplex regions of these primary products of recombination are on the average about 4 kb in length and can include as much as half of the lambda genome.

Bacteriophage lambda↗
PubMed1985

Transformation of yeast with linearized plasmid DNA. Formation of inverted dimers and recombinant plasmid products.

The molecular products of DNA double strand break repair were investigated after transformation of yeast (Saccharomyces cerevisiae) with linearized plasmid DNA. DNA of an autonomous yeast plasmid cleaved to generate free ends lacking homology with the yeast genome, when used in transformation along with sonicated non-homologous carrier DNA, gave rise to transformants with high frequency. Most of these transformants were found to harbor a head-to-head (inverted) dimer of the linearized plasmid. This outcome of transformation contrasts with that observed when the carrier DNA is not present. Transformants occur at a much reduced frequency and harbor either the parent plasmid or a plasmid with deletion at the site of the cleavage. When the linearized plasmid is introduced along with sonicated carrier DNA and a homologous DNA restriction fragment that spans the site of plasmid cleavage, homologous recombination restores the plasmid to its original circular form. Inverted dimer plasmids are not detected. This relationship between homologous recombination and a novel DNA transaction that yields rearrangement could be important to the cell, as the latter could lead to a loss of gene function and lethality.

DNA Repair↗
PubMed1985

On mutation fixation resulting from nitrosoguanidine-induced DNA damage in Escherichia coli K12 ada-.

Brief treatment with nitrosoguanidine of E. coli defective in the removal of the pre-mutagenic lesion, gives rise to cells that segregate mutant clones. No depletion in the number of such cells occurs for at least 4 generations of growth in liquid medium. It is concluded that pre-mutagenic lesions persist and result in copying errors by an otherwise normal replication machinery.

Alkylating Agents↗
PubMed1984

A growth rate distribution model for the age dependence of human cancer incidence: a proposed role for promotion in cancer of the lung and breast.

A biological model is proposed to account for the steep rise of human cancer incidence with age. The model casts in mathematical terms the assumptions that each clone destined to give rise to a detectable tumor displays a characteristic net growth rate and that the assembly of such clones displays a distribution of growth rates. Incidence is introduced as the rate of appearance of clones whose size permits detection. While the cancer formation process may involve a series of stages, we assume that the overall kinetics of tumor detection reflect one stage of development whose duration spans the major portion of the latent period between initial cell alteration and final detection. We further assume that the net growth rates of tumor-forming clones increase in the presence of promotors and resume their original growth rates when the promoting substance is removed. Assuming that cigarette smoke has promoting activity, we show how the model could account for the abrupt impact of cessation of smoking on subsequent lung cancer incidence. If we assume that clones destined to be detected as breast cancers experience promotional activity during the period of a woman's fertility, the model predicts that as a consequence of the slowing down of the clones a discontinuous decline in incidence would follow menopause. Since women experience menopause over a range of ages, we show how aggregating the contributions from these menopausal ages results in an overall age dependence of incidence with no discontinuities and with the observed change in the incidence rate for breast cancer near the age range of menopause.

Age Factors↗
PubMed1984

Evidence for inclusion of regions of nonhomology in heteroduplex products of bacteriophage lambda recombination.

Total intracellular DNA was isolated from replication-restricted bacteriophage lambda crosses in which the infecting parents were heteroallelic for wild-type and deletion mutant alleles. This DNA was examined for the presence of heteroduplex DNA molecules that contained wild-type sequences in one strand and deletion-mutant sequences in the other. Molecules hybrid for a 689-nucleotide deletion in the immunity region of lambda were detected at significant levels only in crosses in which both the red recombination system of lambda and the rec recombination system of Escherichia coli were active. Molecules hybrid for a 1300-nucleotide deletion in the central portion of the lambda genome were detected at significant levels in DNA isolated from both red+ and red- crosses in which recA function was present.

Bacteriophage lambda↗
PubMed1984

Formation of inverted dimer plasmids after transformation of yeast with linearized plasmid DNA.

The formation of an inverted dimer plasmid on transformation with linear molecules is formally analogous to the fusion of the daughters of a broken chromosome at their broken ends. In the latter case, this leads to the formation of a dicentric chromosome, which could break at anaphase. Hence the process is cyclic. Similarly, when our linear molecules are modified by the addition of a cloned yeast centromere, dicentric inverted dimers are not obtained. Instead, we obtain monocentric plasmids with partial duplication and deletion that apparently derive from a process of fusion, bridge-breakage, and fusion. This is not surprising, since it is known that dicentric plasmids undergo breakage in yeast (Mann and Davis 1983). However, any apparent similarity of this process to that which occurs with a broken chromosome in maize must be tempered by the special nature of the transformation process. Most significantly, inverted dimers are rare when sonicated carrier DNA is not present during the transformation. This requirement is not understood, but it is a condition that may not be met in a yeast cell harboring a broken chromosome. It is possible that carrier DNA induces a repair process that results in fusion. On the other hand, a property of the transformation process that results in an inhibition of fusion may be overcome by the presence of carrier DNA. Most inverted dimers are apparently formed from an interaction between two input linear molecules. We cannot rule out the possibility that a minor fraction derive from a single molecule. Thus, the fusion of two input molecules is a much more efficient process than a replicative process that could occur with single linear molecule. For a similar fusion process to occur with a broken yeast chromosome, replication would be required. We do not know if a broken yeast chromosome can replicate. Evidence consistent with the presence of a breakage-fusion-bridge process in yeast has been obtained through the formation of dicentric chromosomes via meiotic recombination (Haber et al. 1984). Spores from these meioses sometimes give rise to a clone that is mixed for markers of the chromosome that could have been dicentric. A process of fusion-bridge-breakage could account for the formation of some of these mixed clones. However, the dicentric chromosomes apparently often survive meiotic disjunction and break in the spore's first mitotic anaphase or possibly in a later generation. Thus, the interpretation of the origin of these mixed clones is uncertain. Some aspects of the fusion process are especially intriguing.(ABSTRACT TRUNCATED AT 400 WORDS)

Chromosome Mapping↗
PubMed1984

Acute acalculous cholecystitis.

Sixty-eight patients with acute acalculous cholecystitis were reviewed. The results of history and physical examinations were usually nondiagnostic. IDA cholescintigraphy (93 per cent accuracy rate) was the only reliable diagnostic modality. The results of oral cholecystography, intravenous cholangiography and ultrasonography were considerably less reliable. One-half of the patients had gangrenous cholecystitis. Cholecystectomy was the preferred operation with an over-all mortality of 9 per cent. IDA cholescintigraphy is an important new modality for the diagnosis of acute acalculous cholecystitis which, in the past, has often been difficult to diagnose.

Acute Disease↗
PubMed1983

Detection of non-homology-containing heteroduplex molecules.

Heteroduplex DNA molecules which contain both the wild-type and mutant sequences of a deletion nonhomology possess a characteristic electrophoretic mobility in agarose and can be readily separated from both the wild-type and deletion-containing parental homoduplex fragments. Because of the partial single stranded character of these deletion-containing heteroduplex molecules, they are selectively bound to nitrocellulose filters, and once bound, can be selectively detected by hybridization with radioactively labeled single-stranded DNA which is homologous to the sequences absent in the deletion mutation.

Bacteriophage lambda↗
PubMed1983

A screening procedure for the efficient recognition of Escherichia coli K-12 mutants.

A method is described for the rapid screening of large numbers of E. coli colonies for detection of auxotrophs and mutants defective in sugar or succinate metabolism. The procedure permits recognition of forward mutations in a large number of functions and is applicable to the isolation of temperature-sensitive mutants in essential and nonessential genes.

Escherichia coli↗
PubMed1983

Effects of nonhomology on bacteriophage lambda recombination.

When crosses are performed under conditions severely restricting DNA synthesis, the presence of DNA sequence nonhomologies in the lac region of lambda plac5 limits the parental material contribution to and the yield of phage recombinant in a region bracketed by the nonhomologies. These observations are consistent with the expectation of a role for branch migration in the formation of heteroduplex structures under these conditions. Under conditions permissive for DNA replication, bracketing a region with nonhomologies has an only modest effect on the yield of recombinants within that interval. In addition, recombinants within such a bracketed interval manifest an excess of coincident exchange events in an adjacent region. These observations suggest the possibility that, under conditions permissive for DNA replication, regions of nonhomology can be included in heteroduplex structures.

Bacteriophage lambda↗
PubMed1983

The role of technetium-99m iminodiacetic acid (IDA) cholescintigraphy in acute acalculous cholecystitis.

Technetium-99m iminodiacetic acid (IDA) cholescintigraphy was performed in 15 patients with acute acalculous cholecystitis. Fourteen of the 15 patients with acute disease had positive findings, indicating the presence of cystic duct or common duct obstruction. One case in which the gallbladder was visualized failed to respond to sincalide stimulation; this was classified as a suggestive finding of disease. The diagnostic accuracy of 99mTc-IDA cholescintigraphy was far superior to the other imaging studies used (8 sonograms, 1 intravenous cholangiogram, 3 oral cholecystograms, 1 percutaneous transhepatic cholangiogram). The 99mTc-IDA study is recommended as the imaging procedure of choice for examining patients with suspected acute acalculous cholecystitis.

Acute Disease↗
PubMed1980

Workmen's compensation hearing loss claims.

This presentation deals with the evaluation of occupational hearing loss claims, the tests used over the years to determine the hearing status of the claimants, and the various methods used to convert the results of the audiometric tests in terms of percentage of hearing impairment. A significant modification of the 1961 American Medical Association formula has recently been adopted by the Board of Directors of both the American Academy of Otolaryngology and the American Council of Otolaryngology. The modified formula has been published: "Guide for the Evaluation of Hearing Handicap," JAMA, 241, (19): May 11, 1979. The reasons for this modification and its application in the evaluation of occupational hearing loss claims are discussed. Otolaryngologists are urged to become acquainted with the modified AAO formula and to apply it in the evaluation of hearing loss claims.

American Medical Association↗
PubMed1979

On the diagnosis and treatment of breast cancer.

Randomized trials comparing surgical treatments of breast cancer show that radical mastectomy offers no greater benefit than simple mastectomy followed by radiotherapy. Furthermore, in terms of survival, radical mastectomy seems to be no better than wide excision followed by radiotherapy when the disease is clinically diagnosed as stage 1. The incidence of diagnosed breast cancer showed an 18% increase between 1935 and 1965 and a 50% increase between 1965 and 1975. However, breast cancer mortality has remained unchanged for at least the past 40 years. Analysis of survival curves of women with breast cancer suggests that two or more populations exist, with about 40% suffering fatal outcome unaffected by treatment. The remaining 60% exhibit a relative mortality only modestly different from that of women of similar ages without evidence of disease. Increasing detection of an entity that is histologically defined as malignant but biologically relatively benign could account for the observed increase in incidence.

Breast Neoplasms↗