T7 exonuclease (gene 6) is necessary for molecular recombination of bacteriophage T7

The role of T7-induced exonuclease (gene 6) in molecular recombination was studied by examining the fate of parental DNA during parental-to-progeny recombination. The method used was to compare infections with T7(+), T7am-6-233 (am gene 6), or T7ts6-136 (ts gene 6) under permissive and nonpermissive conditions. CsCl density gradient analysis of replicative DNA indicated that T7 exonuclease is necessary for recombination to occur, i.e., in the absence of the exonuclease the parental DNA replicated continuously as a hybrid molecule and did not recombine. Further studies under conditions where replicative DNA was denatured and analyzed by CsCl density gradient centrifugation indicated that the exonuclease is also needed for a limited amount of covalent repair of recombinants. A repair function for the T7-induced exonuclease is also suggested by results obtained from alkaline sucrose gradient analysis of replicative DNA. Under conditions nonpermissive for the exonuclease, discontinuities in the DNA accumulated during infection by T7am6-233 or by T7ts6-136.

Comparison of oral health in four Canadian Indian communities

Dental caries and periodontal disease experience varied considerably among four populations of Indian communities although environmental conditions seemed similar.

Analysis of submicroscopic structures by their emitted x-rays

X-ray emission spectra have been recorded from several biologic tissues using a multichannel energy-dispersive analyzer with a retractible semiconductor detector coupled to a Cambridge Mark II scanning electron microscope. Particular attention has been given to the detection of silver in experimental argyria, of calcium in dermoid scales and in experimental necrosis of the kidney and of sulfur in the inner and outer portions of reptilian skin. Sulfur and chlorine have been found associated with silver in argyria. Phosphorus was associated with calcium both in the dermal scales and in necrotic areas. In addition to these elements, trace amounts of copper, zinc, lead, sodium, potassium, iron, arsenic, osmium and uranium have been detected in various normal and experimental situations. The applicability of the combined instrument to cytochemical problems is briefly discussed.

The psycho-social consequences of aircraft noise

The paper describes and compares various methods of investigating the social disamentity caused by aircraft noise. These methods include social surveys,the analysis of complaints, and direct observation. Each has practical, political and statistical advantages and disadvantages. The effects of aircraft noise are also discussed and the conclusion is drawn that if social pressure continues to be applied, the technologists can and will quieten aircraft engines.