Functions of DNA Helicases in the DNA Metabolism of Escherichia Coli

DNA helicases catalyze the separation of double-stranded DNA into single strands using the energy of ATP hydrolysis. The four helicases which have been found in Escherichia coli are listed in Table 1. These enzymes are the helicases I, II, III and the helicase specified by the rep gene. As the table shows the four proteins differ considerably with respect to Mr, number of molecules per cell and, in particular, mechanism of action. Each of the helicases requires a region of single-stranded DNA to initiate unwinding, and each of the four enzymes unwinds DNA unidirectionally relative to the chemical polarity of the DNA strand to which it is bound. The direction of unwinding depends on the nature of the helicase

UvrD helicase, unlike Rep helicase, dismantles RecA nucleoprotein filaments in Escherichia coli

The roles of UvrD and Rep DNA helicases of Escherichia coli are not yet fully understood. In particular, the reason for rep uvrD double mutant lethality remains obscure. We reported earlier that mutations in recF, recO or recR genes suppress the lethality of uvrD rep, and proposed that an essential activity common to UvrD and Rep is either to participate in the removal of toxic recombination intermediates or to favour the proper progression of replication. Here, we show that UvrD, but not Rep, directly prevents homologous recombination in vivo. In addition to RecFOR, we provide evidence that RecA contributes to toxicity in the rep uvrD mutant. In vitro, UvrD dismantles the RecA nucleoprotein filament, while Rep has only a marginal activity. We conclude that UvrD and Rep do not share a common activity that is essential in vivo: while Rep appears to act at the replication stage, UvrD plays a role of RecA nucleoprotein filament remover. This activity of UvrD is similar to that of the yeast Srs2 helicase