The alleviation of DNA restriction during the SOS response in Escherichia coli has been further investigated. With the EcoK DNA restriction system UV irradiated wild-type cells show a 10(4)-fold increase in ability to plate non-modified lambda phage and a 3-4 fold increase in transformation by non-modified plasmid DNA. A role for the umuDC genes of E coli in the process of SOS-induced restriction alleviation was identified by showing that a umuC122::Tn5 mutant could alleviate EcoK restriction to only 5% that of wild-type levels. Although umuDC are better characterized for their pivotal role in SOS induced mutagenesis, it is demonstrated here that umu-dependent alleviation of EcoK restriction is a transient process in which umu-dependent mutagenesis plays little part. A second form of SOS induced alleviation of DNA restriction is described in this paper involving the McrA restriction system. The mcrA gene is shown to be encoded within a defective prophage called e14 situated at the 25 min region on the Escherichia coli genetic map. e14 is known to abortively excise from the chromosome after SOS induction and it is demonstrated in this report that mcrA is lost from the genome after SOS induction as part of e14. This results in co-ordinate decrease in the level of McrA restriction within a population of cells.