Effect of  -Radiation on the Donor Ability of recA and recA+ Strains of Escherichia coli

Abstract

S U M M A R YThe sensitivity to y-radiation of the chromosome of a r e d + Hfr strain of Escherichia coli was six times greater when assayed by donor ability in conjugation than when assayed by survival in unmated bacteria. With a recA Hfr strain sensitivities were comparable when assayed by the two methods, and were similar to the sensitivity of donor ability in the r e d + strain. We suggest that the greater resistance of survival in the r e d + strain may be due to apost-replicational repair process which does not occur in the recA strain and which is prevented from occurring when a chromosome of either strain is involved in conjugation. When a recA Hfr and a recA F-strain were given 8.5 krads y-radiation, enough to inhibit completely normal DNA synthesis, the DNA synthesis associated with conjugation still occurred. This suggests that the lesions which prevent recombinant formation may act not by inhibiting chromosome transfer, but by affecting a later stage in the process of integration.

Effect of tif Expression, Irradiation of Recipient and Presence of Plasmid pKM101 on Recovery of a Marker from a Donor Exposed to Ultraviolet Light Prior to Conjugation

“…Escherichia coli JC5088 recA str+ Hfr (Green et al, 1971) was used as the donor in all the experiments. Its origin of transfer is at 62 min and the his gene (at 44 min) enters after about 20 % of the genome has been introduced.…”

DNA Synthesis in Gamma-irradiated Recombination Deficient Strains of Escherichia coli

“…Moreover, Green, Bridges & Riazuddin (1971) have shown that under conditions where normal DNA synthesis is totally inhibited by gamma-irradiation in r e d male and female strains, conjugation DNA synthesis may still be detected. This implies that if DNA synthesis were occurring in gamma-irradiated r e d strains it would be detected.…”

“…Although it is not known whether this slow repair process occurs before, during or after DNA replication, its slowness suggests that gaps could persist until replication. Moreover, the effects of gamma-irradiation on conjugation (Green, Bridges & Riazuddin, 1971) suggest that red+-dependent repair is completed during or after replication of DNA bearing the lesion. Cairns & Davern (I 966), however, have found that one 32P-induced double strand break anywhere in the chromosome is sufficient to inhibit DNA synthesis immediately in a presumed recA strain and this result would imply that recA+-dependent repair of 32P damage must occur before DNA replication can continue.…”

Gamma ray mutagenesis in a strain of Escherichia coli deficient in DNA polymerase I

“…Although the recA gene also affects genetic recombination, it would appear that recombination between existing chromosomes is not necessary for RecA+dependent repair of single strand DNA breaks induced by ionising radiation (Bridges, 1971). Moreo'ver, there is evidence that such breaks may remain unrepaired until the subsequent DNA replication (Bridges, Green and Munson, 1971;Green, Bridges and Riazuddin, 1971;Hudnik-Plevnik and Djorgevic, 1971). Another pathway for the repair of single-strand breaks has recently been demonstrated by Town, Smith and Kaplan (1971) to depend on DNA polymerase I (Kornberg enzyme).…”