One of the crucial problems in radiation protection is the reality of the negligible dose or de minimus concept (1-4). This issue of a "practical zero" and its resolution is central to our understanding of the controversy concerning the existence of a "safe" dose in radiological health. However, for very low levels of environmental mutagens and carcinogens including low doses of low-LET radiations (less than 1 cGy or 1 rad), spontaneous or endogenous DNA damage may have an increasing impact on the biological consequences of the induced cellular response. It is this issue that is addressed in this communication. The following discussion is intentionally limited to a comparison of low-LET radiation since its effects are due primarily to indirect damage in cellular DNA brought about by OH radicals. Indirect effects of low-LET radiation under aerobic conditions are reported to account for 50-85% of measured radiation damage in cells (5, 6). High-LET radiation, on the other hand, produces unique DNA damage (7) primarily by direct effects (5) which is less likely to be properly repaired (7). Spontaneous or intrinsic modification of cellular DNA is ubiquitous in nature and likely to be a major cause of background mutations (8), cancer (9), and other diseases (10). The documentation of this intrinsic DNA decay has increased at a rapid pace in recent years and has not gone unnoticed by contemporary radiobiologists. Setlow (11) and more recently Saul and Ames (12) summarized the findings of Lindahl and Karlstrom (13) and others (14) which suggest that approximately 10,000 measurable DNA

The observation of recovery of living cells from the effect of ionizing radiations has been reported repeatedly. The literature concerning this effect has been reviewed by Lea ('47) and more recently by Gray ('52) and Allen et al. ('51). Revera1 techniques have been employed to demonstrate recovery in living organisms, eg., comparison of the efficiency of single and fractionated doses of a radiation or comparison of the efficiency of a radiation applied at high and low intensities. Tlie use of these techniques has yielded information coiicerning the recovery which occurs during the course of radiation exposure. A review of the literature (Lea, '47) indicates that bacteria irradiated in aqueous suspensions do not recover during the exposure to ionizing radiations. Some types of cells a r e able to recover from the effects of ionizing radiations if maintained in the cold or in a starved condition after irradiation (Strangeways and Fell, '27 ; Spear and Gliicksmann, '39; Cook, '39; Allen et al., '51). I n some of the cases reported, however, apparent recovery was in fact merely a postponement of the radiation effect, which was shown to take its normal course when active metabolism and growth were resumed. Latarjet ('43) showed that holding bacterial cells on nutrient agar a t refrigerator temperatures

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3 Complex medium consists of the following: tryptone (1 per cent), yeast extract (1 per cent), K2HPO4 (0.5 per cent), and glucose (1 per cent).

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During studies on the metabolism of Escherichia coli (Texas), it was found that hydrogenase was either entirely absent or present in relatively small quantity in cells harvested from a glucose mineral salts medium. This observation suggested a nutritional approach to the study of this enzyme, and the results herein presented show clearly that the production of hydrogenase during the growth of E. coli is conditioned by the presence of certain amino acids. METHODS E. coli (Texas) was carried by serial transfer on a basal medium of the following composition: 0.1 per cent KH2PO4, 0.07 per cent MgSO4-7H20, 0.1 per cent NaCl, 0.4 per cent (NH4)2HPO4, 0.05 per cent citric acid neutralized with 10 per cent NaOH, 0.00005 per cent FeCls, and 1 per cent glucose added aseptically. This medium was adjusted to pH 6 to 7 before sterilization. One ml of a 24-hour culture of E. coli was introduced as inoculum into 100 to 250 ml of the growth medium, and the cultures were incubated at 37 C for 21 hours. The cells were harvested by centrifugation, washed in distilled water, recentrifuged, and suspended in 1 per cent NaCl. The concentration of cells was determined in terms of bacterial nitrogen per ml by measuring turbidity in a Klett-Suxmmerson photoelectric colorimeter and converting into terms of nitrogen content by the use of a previously standardized table. Hydrogenase activity of the washed cell suspensions was measured by hydrogen uptake in a standard Warburg apparatus with the bath temperature at 38 C in an atmosphere of hydrogen. Methylene blue was used as the substrate and was tipped in from the side arm after equilibration at 38 C. The main compartment contained the washed celLs, 0.2 M phosphate buffer at pH 6, and was brought to a volume of 3 ml with distilled water. EXPERIMENTAL RESULTS

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Billen, Daniel (University of Texas M. D. Anderson Hospital and Tumor Institute, Houston, Tex.), and Roger Hewitt . Influence of starvation for methionine and other amino acids on subsequent bacterial deoxyribonucleic acid replication. J. Bacteriol. 92: 609–617. 1966.—A study has been made of the subsequent replicative fate of deoxyribonucleic acid (DNA) synthesized during amino acid starvation by several multiauxotrophic strains of Escherichia coli . Using radioisotopic and density labels and a procedure whereby total cellular DNA is analyzed, we have confirmed and extended a recent report that the DNA made during amino acid starvation behaves anomalously during subsequent DNA replication. When 5-bromouracil (BU) serves as the density lable, 40% or more of the DNA synthesized during starvation will subsequently fail to replicate during three cell generations. Selective amino acid effects were noted. In two methionine-requiring bacteria, methionine deprivation appeared to be of singular importance in influencing the subsequent replicative fate of the DNA made in its absence. When a non-BU density label (N 15 , C 13 ) was utilized, the effects of amino acid starvation were less obvious. Although the DNA synthesized during complete amino acid starvation in a methionine-requiring E. coli was subsequently more slowly replicated, most of the DNA was finally duplicated during three generations of growth. If methionine was present during starvation for other required amino acids, the subsequent replication rate of the DNA synthesized during this time was more nearly normal, and complete replication was observed. The results have been interpreted as indicating that DNA synthesized during amino acid starvation, and especially during methionine starvation, is somehow altered, and that BU substitution for thymine may interfere with the restoration of such DNA to its replicative state.