Sensitization to Radiation by Loss of Recombination Ability in a Temperature-sensitive DNA Mutant of Micrococcus radiodurans Held at Its Restrictive Temperature

Abstract

SUMMARYTwo temperature-sensitive mutants of Micrococcus radiodurans defective in DNA synthesis, which were very resistant to the lethal effect of ultraviolet and ionizing radiation at permissive temperatures, became sensitive to radiation and also to the action of N-methyl-N'-nitro-N-nitrosoguanidine when held at the restrictive temperature of 39 "C. With M. radiodurans tsr the sensitization began soon after transfer to 39 "C and reached a maximum 4 h later. During this period there was no loss of viability. After 4 h the shoulders of the ultraviolet and ionizing radiation survival curves had almost completely disappeared and the exponential part of the curves had doubled in slope. The size of the shoulder fell exponentially with the time the bacteria were held at 39 "C. Sensitization occurred in the presence of chloramphenicol. During the period the bacteria were held at 39 "C their ability to effect recombination as measured by transformation fell exponentially and was correlated with the rate of loss of the shoulder. This suggests that the repair which gives rise to the large shoulders of the radiation survival curves is of the post replication recombination type.The recovery of radiation resistance at 30 "C in bacteria which had been exposed to 39 "C for 75 min did not begin immediately. For 55 min there was no measurable increase in resistance but after 75 min substantial recovery had occurred and by 105 min was complete. Recovery of resistance did not occur in the presence of chloramphenicol even when the chloramphenicol was added 30 min after the bacteria had been at 30 "C.The sensitization to radiation was not a general property of temperaturesensitive (ts) mutants.

Oxidative Stress Resistance inDeinococcus radiodurans

“…A reduced frequency of recombination in a recA mutant is commensurate with the reduction in radiation resistance (445), while the shoulder of the survival curve is completely eliminated for ionizing radiation, UV radiation, and MMC (223,439) (Fig. 8).…”

“…Hydroxylamine and nitrous acid cause the deamination of adenine and cytosine. D. radiodurans is also very resistant to 4-nitroquinoline-N-oxide (4NQO), which has lethal and mutagenic effects on E. coli similar to those of UV radiation (445). The exposure of D. radiodurans to 50 g/ml 4NQO for 4 h does not reduce viability or induce mutations, whereas an equivalent exposure of E. coli reduces viability to 0.1% (445).…”

“…The repair of SSBs in D. radiodurans can be monitored by using alkaline sucrose gradients (79,154,157,350). The repair of DSBs is more difficult than the damage that affects a single strand (SSB or base damage) and relies upon interchromosomal recombination (125,445,572,676).…”

“…The major BPP in UV-irradiated D. radiodurans is CPD TC (47.2%), while 6-4 PPs are least represented (428). UVinduced DNA damage is repaired by two nucleotide excision repair mechanisms, UvrABC and UVDE (63,413,441), and a genetic recombination mechanism (439,445). Accordingly, the most UV-sensitive DNA repair mutants in D. radiodurans are recA, recO, polA, and uvrA uvsE mutants (171, 223-225, 434, 439, 441, 603, 665) (Fig.…”

“…As in E. coli, the repair of UV-induced DNA damage in D. radiodurans therefore relies on excision (63) and recombination (439,445) mechanisms. The completion of excision repair precedes the completion of recombinational repair (440).…”