GENETIC AND MOLECULAR ANALYSES OF UV RADIATION-INDUCED MUTATIONS IN THE FEM-3 GENE OF CAENORHABDITIS ELEGANS

The utility of a new target gene (fem-3) is described for investigating the molecular nature of mutagenesis in the nematode Caenorhabditis elegans. As a principal attribute, this system allows for the selection, maintenance and molecular analysis of any type of mutation that disrupts the gene, including deletions. In this study, 86 mutant strains were isolated, of which 79 proved to have mutations in fem-3. Twenty of these originally tested as homozygous inviable. Homozygous inviability was expected, as Stewart and coworkers had previously observed that, unlike in other organisms, most UV radiation-induced mutations in C. elegans are chromosomal rearrangements of deficiencies (Mutat. Res. 249 , 37–54, 1991). However, additional data, including Southern blot analyses on 48 of the strains, indicated that most of the UV radiation-induced fem-3 mutations were not deficiencies, as originally inferred from their homozygous inviability. Instead, the lethals were most likely “coincident mutations” in linked, essential genes that were concomitantly induced. As such, they were lost owing to genetic recombination during stock maintenance. As in mammalian cells, yeast and bacteria, the frequency of coincident mutations was much higher than would be predicted by chance.     

D-c electric-potential method applied to thermal/mechanical fatigue crack growth

The paper describes a d-c electric potential system for measuring crack length under thermal/mechanical fatigue-crack-growth (TMFCG) test conditions. A programmable d-c current supply and precision multimeter produce reliable electric-potential readings. H.H. Johnson's formula is used to calculate crack length from electric potential for the center-crack-tensionM(T) geometry. Calibration constants for the formula are determined from an initial optical crack-length measurement. The resolution of the system is 1.0 microvolt which corresponds to a crack extension of approximately 0.002 mm for the center-crack-tension geometry using a current of 10.00 amps. Good crack-length accuracy and low data scatter are achieved by taking special precautions to minimize or eliminate errors in potential measurement due to thermal effects. Material resistivity changes are identified as the cause of short and long term changes in the measured electric potential for uncracked specimens. Crack-length accuracy is discussed in terms of short-term scatter and longterm drift.     

N.M.R. studies of liquid boron trifluoride

Previous ¹⁹F N.M.R. studies of liquid BF₃ have been extended by obtaining the ¹¹B spectrum of BF₃ and the ¹⁹F spectrum of ¹⁰BF₃ at various temperatures. Values for the B-F coupling constant and the boron relaxation times have been obtained by computer matching and visual matching of observed and calculated spectra. Relaxation times have the Arrhenius temperature dependence found previously. The temperature dependence of the B-F coupling constant is discussed. Some of the possible advantages of ‘high spin spectra’ are discussed.     

Astronomical gamma ray telescopes in the pair production regime

Most of the gamma ray telescopes used up to now for energies above about 50 MeV have been very similar in design. Anticipated requirements for future telescopes will probably necessitate the use of a considerable amount of new technology.     

On Greene-Kleitman's theorem for general digraphs

Linial conjectured that Greene—Kleitman's theorem can be extended to general digraphs. We prove a stronger conjecture of Berge for digraphs having k-optimal path partitions consisting of ‘long’ paths. The same method yields known results for acyclic digraphs, and extensions of various theorems of Greene and Frank to acyclic digraphs.