The resistance of Micrococcus radiodurans to killing and mutation by agents which damage DNA

An exonuclease activity associated with DNA polymerase I of Micrococcus radiodurans

An exonuclease activity is associated with one of three DNA polymerase in Micrococcus radiodurans. The nuclease activity co-sedimented with its DNA polymerase I of this bacterium on glycerol gradient centrifugation. Both activities show the same optimum pH and heat-inactivation kinetics. This nuclease hydrolyzes preferentially double-stranded DNA in an exonucleolytic manner from both ends of the duplex DNA. The products of hydrolysis are mostly deoxyribonucleoside 5'-monophosphate and no nucleosides are released into the acid-soluble fraction. Di- or other oligonucleotides are also produced but their relative amounts are constant during the time of incubation. The exonuclease activity requires Mg2+ and is inhibited by high concentrations of KCl as is DNA polymerase I of M. radiodurans.

Multiplicity of genome equivalents in the radiation-resistant bacterium Micrococcus radiodurans

The complexity of the genome of Micrococcus radiodurans was determined to be (2.0 +/- 0.3) X 10(9) daltons by DNA renaturation kinetics. The number of genome equivalents of DNA per cell was calculated from the complexity and the content of DNA. A lower limit of four genome equivalents per cell was approached with decreasing growth rate. Thus, no haploid stage appeared to be realized in this organism. The replication time was estimated from the kinetics and amount of residual DNA synthesis after inhibiting initiation of new rounds of replication. From this, the redundancy of terminal genetic markers was calculated to vary with growth rate from four to approximately eight copies per cell. All genetic material, including the least abundant, is thus multiply represented in each cell. The potential significance of the maintenance in each cell of multiple gene copies is discussed in relation to the extreme radiation resistance of M. radiodurans.

The relation of repair phenomena to mutation induction in bacteria

The relation of various processes to mutation induction by radiation and chemicals is discussed for for various species of bacteria. A variety of repair processes have been identified at the molecular level that can eliminate many kinds of potentially mutagenic lesions before they can be converted to final mutation. Fixation often but not always occurs at replication. A number of mutagens, including UV light, ionizing radiation, and a number of chemicals, induce an error-prone process, perhaps a modification of the proof-reading system, that allows bacteria to survive after potentially lethal damage at the expense of making errors. Some mutagens, notably monofunctional alkylating agents and base analogues, produce mutations by other processes. Even in these cases, repair processes play an important role. There is some evidence that error-free as well as error-prone repair processes can be induced. A brief discussion is given of the relation of these findings to the practical problems of hazards estimations.

Minimal requirements in defined media for improved growth of some radio-resistant pink tetracocci

Defined media permitting extensive growth of representative pink radio-resistant tetracocci (Micrococcus radiodurans, Micrococcus roseus, and Micrococcus radiophilus) and two controls (an ultraviolet-sensitive mutant of M. radiodurans and Micrococcus luteus) are described. Availability of Fe (especially Fe3+) proved essential for good growth, as evidenced by (i) favorable effects of hydroxamic acids, e.g., salicylhydroxamic acid, and (ii) the growth promotion by hemin when joined with elevated concentrations of Fe. Cobalamin (B12) and methionine were interchangeable as an absolute requirement for methionine not affected by B12. M. luteus required neither. Pink radio-resistant micrococci may form a coherent group. Some divergences among them might be attributable to the method for isolating them, which for ordinary bacteria would be mutagenic to the point of total lethality. The ecology of these tetracocci vis-à-vis other pink-red radio-resistant organisms is discussed in relation to a question: can these bacteria be isolated without dependence on radiation as the cardinal selective factor?

Separation of DNA-dependent DNA polymerase activities in Micrococcus radiodurans

DNA polymerase activities in Micrococcus radiodurans were separated into two fractions after purification more than 2000 fold. They differ in pH optimum and residual activities in the absence of a full deoxyribonucleoside triphosphates complement. NAD partly inhibited one of the activities. Both activities were eluted as a single peak on gel filtration and sedimented at the same rate on glycerol gradient centrifugation. Molecular weight 140000 was calculated from Stokes radius and sedimentation constant. Deoxyribonuclease activity was detected on one of the polymerase activities which preferentially degraded double-stranded DNA. Priming activity of nicked DNA was reduced by gamma-irradiation. These results have been related to the possible rolls in repair synthesis in vivo or DNA synthesis in permeable cells of M. radiodurans.

Lethla effect of nitrous acid on Escherichia coli

The effect of nitrous acid (NA) on viability, integrity of cellular DNA and on membrane transport were studied in 5 strains of Escherichia coli. Stationary phase cells, grown on mineral salts medium, were exposed to NA. The viability of strains decreased in thefollowing order: W3110 wild-type greater than WP2 wild-type, WP2 uvrA greater than NG30 recA greater than P3478 polA. Alterations were found in the DNA sedimentation profile in alkaline sucrose gradient. Disturbance of DNA synthesis was measured by 3H-labelled thymidine ([3H]Thd) incorporation. No degradation of DNA was found after NA treatment. Low doses of NA caused significant inhibition of leucine and glucose transport into whole cells. The results are interpreted in terms of the multi-target action of NA causing the death of cells.