RecA Activity Research Articles

Effects of the rec and exr mutations of Escherichia coli on UV reactivation of bacteriophage lambda damaged by different agents

• We confirm that UV reactivation of UV-damaged phage λ does not occur in recA or exr mutants of Escherichia coli. Irradiation of recA and exr bacteria also reduces their ability to perform host-cell reactivation to a much greater extent than wild-type bacteria. RecB and recC mutations have no effect, neither have the phage mutations int and red. • When phage λ is damaged by HNO 2, NH 2OH or nitrogen mustard, UV reactivation occurs in recA and exr mutants. UV-irradiated phage λ in which 5-bromouracil (5-BU) has been incroporated is UV-reactivated in recA but not in exr bacteria. • The breakdown of 32P-labelled phage λ has been followed under different combinations of host and phage irradiation for the different host bacteria. Irradiation of wild-type bacteria depressed the breakdown of irradiated phage. In contrast, breakdown of irradiated or unirradiated phage was increased by irradiation of recA or exr host bacteria. This increase may be analogous to the uncontrolled breakdown of bacterial DNA which occurs after irradiation of recA bacteria. Breakdown itself is not responsible for the elimination of UV reactivation in recA bacteria since the recABC mutant did not show excessive breakdown but has recA phenotype in UV reactivation. • Phage mutagenesis, which normally accompanies UV reactivation, did not occur in recA or exr bacteria although HNO 2-damaged phage was UV-reactivated in the mutant hosts. Clear plaque mutagenesis is not an essential feature of UV reactivation but is linked with recA and exr gene activity. • It is concluded that the recA and exr gene products are not directly involved in UV reactivation, but may be necessary to stabilise or modify UV lesions in phage DNA before reactivation can take place. We have not shown any connection between this activity and clear plaque mutagenesis. Absence of recA or exr gene activity renders UV lesions unrepairable by UV reactivation and, to some extent, by host-cell reactivation. Breakdown of the DNA follows. The different results obtained with different kinds of damage must reflect specificity of the recA and exr gene products for particular types of lesion.