Role of DnaB Helicase in UV-Induced Illegitimate Recombination in Escherichia coli

Illegitimate recombination (IR) takes place between DNA sequences that have little or no homology and is a major cause of chromosomal aberrations, such as deletions or translocations. IR can be classified into two types, short-homology-independent IR (SHIIR) and short-homology-dependent IR (SHDIR). SHIIR occurs between sequences having virtually no homology and is mediated by DNA topoisomerases (2, 3, 24). SHDIR is induced by UV irradiation or other DNA-damaging agents and requires short regions of homology between recombination sites (24, 25, 28). SHDIR usually takes place at a low frequency, but it is greatly enhanced by genotoxic agents (11, 18). It is known that this type of recombination is enhanced by RecE and RecJ and is suppressed by DNA polymerase I, SbcB, RecQ, and UvrAB (1, 7, 9, 10, 25, 27). Previous studies have suggested that DNA ends are processed by 5′-to-3′ exonucleases, such as RecE or RecJ, and as a result DNAs with a 3′ overhang are formed. These molecules anneal with other molecules carrying a complementary overhang, and then end joining takes place. It has also been found that DNA-binding proteins affect this type of IR. Fis and IHF (integration host factor) enhance SHDIR, and H-NS suppresses it (21, 22). It is known that a double-stranded break (DSB) can result from arrest of a replication fork, which may be caused by DNA secondary structure, DNA lesions, or DNA-bound proteins (5, 6, 8). The involvement of Rep and DnaB helicases in DSB formation indicates that there is a link between DNA replication stoppage and the formation of DSBs (13). In addition, RuvABC, which catalyze branch migration and cleavage of Holliday junctions, are responsible for DSB formation at stalled replication forks (20). In this study, to determine the role of DNA replication in UV-induced IR, we examined the effects of dnaB(Ts) mutations on IR. One of the mutants used, a dnaB14 mutant, had a defect in the elongation process during DNA replication (12), and another mutant, a dnaB252 mutant, had a defect in initiation of DNA replication (19). We found that the frequency of UV-induced IR was reduced by the elongation-deficient mutation in the dnaB14 mutant but not by the initiation-deficient mutation in the dnaB252 mutant. Thus, we concluded that processivity of DnaB helicase is a prerequisite for UV-induced IR. Below we discuss a model for the possible function of DnaB in UV-induced IR.