Repair-defective mutants of Alteromonas espejiana, the host for bacteriophage PM2

Consequences and repair of radiation-induced DNA damage: fifty years of fun questions and answers

PURPOSE

To summarize succinctly the 50 years of research undertaken in my laboratory and to provide an overview of my career in science. It is certainly a privilege to have been asked by Carmel Mothersill and Penny Jeggo to contribute to this special issue of the International Journal of Radiation Biology focusing on the work of women in the radiation sciences.

CONCLUSION

My students, post-docs and I identified and characterized a number of the enzymes that recognize and remove radiation-damaged DNA bases, the DNA glycosylases, which are the first enzymes in the Base Excision Repair (BER) pathway. Although this pathway actually evolved to repair oxidative and other endogenous DNA damages, it is also responsible for removing the vast majority of radiation-induced DNA damages including base damages, alkali-labile lesions and single strand breaks. However, because of its high efficiency, attempted BER of clustered lesions produced by ionizing radiation, can have disastrous effects on cellular DNA. We also evaluated the potential biological consequences of many of the radiation-induced DNA lesions. In addition, with collaborators, we employed computational techniques, x-ray crystallography and single molecule approaches to answer many questions at the molecular level.

Maltose adaptive mutant of heterotrophic marine bacterium, Alteromonas espejiana Bal-31: changes in the growth and the induction of extracellular alpha-amylase

Growth of the heterotrophic marine bacterium, Alteromonas espejiana Bal-31 was inhibited in the presence of sucrose, maltose and even glucose, but not with starch. Extracellular alpha-amylase was induced with a lag phase of 2 h in the presence of starch. In contrast, cell growth of the S2a mutant was not affected by the addition of maltose, and starch was ineffective in the induction of extracellular alpha-amylase in this mutant. Activity of extracellular alpha-amylase was induced from the S2a mutant with a 4-h lag phase in the presence of maltose, and the high level of enzyme activity was maintained for at least 24 h. Activity of alpha-amylase induced by both wild type starch and S2a mutant maltose cultures were mainly observed in extracellular locations. This activity could be stopped by tetracycline treatment, indicating that enzyme induction was dependant on gene expression and not on enzyme protein secretory mechanisms. Our results showed that the mutation in S2a changed the growth and the modulation of the specific alpha-amylase in response to carbon nutrients.

Presence of inducible DNA repair in Bacillus thuringiensis

Weigle reactivation of ultraviolet-irradiated luminal diameter 8 bacteriophage was observed after ultraviolet treatment of Bacillus thuringiensis cells. A slight increased frequency of clear plaque mutants was detected among the survivors. The kinetics of induction of the phage reactivation and phage mutagenesis have been determined. The presence of chloramphenicol before and after irradiation abolished the induction of repair and mutagenesis. These experiments suggest that, in spite of the relatively small mutagenic response in bacteriophage progeny, B. thuringiensis has an inducible repair system responsible to the significant Weigle reactivation of irradiated phage.

The role of specific DNA base damages in the X-ray-induced inactivation of bacteriophage PM2

Two types of X-ray-induced base damages, alkali-labile sites and thymine ring saturation products, were quantitated in PM2 DNA irradiated in the phage capsid under oxic and anoxic conditions. The extent of formation of these base damages was compared with the number of single- and double-strand breaks and lethal hits produced under the same conditions. The individual inactivation efficiencies of alkali-labile sites and thymine ring saturation products were determined by selectively inducing each of these damages in isolated PM2 DNA by chemical means in vitro and determining the rate of biological inactivation of the treated DNA by transfection. For each lethal X-ray hit induced in oxic conditions there were 1.06 alkali-labile sites, 0.40 thymine ring saturation products, 2.09 singe-strand breaks and 0.11 double-strand breaks in the PM2 genome. In anoxic conditions, the respective number of lesions was 1.00, 0.19, 1.73 and 0.09. The individual inactivation efficiencies of thymine ring saturation products and alkali-labile sites were found to be essentially equal, 7-8 lesions per lethal event in the PM2 genome. Alkali-labile sites and thymine ring saturation products together accounted for 15-20% of the biological inactivation of X-irradiated bacteriophage PM2. The presence or absence of oxygen during irradiation did not affect the contribution to inactivation made by alkali-labile sites, but the contribution by thymine ring saturation products to inactivation was about 2-fold higher in oxic compared with anoxic conditions. With the 4 lesions measured, we have accounted for some 28-34% of the lethal events in X-irradiated PM2 phage, most of the remaining events being caused by as yet unidentified base damages.