Biochemical basis of hyper-recombinogenic activity of Pseudomonas aeruginosa RecA protein in Escherichia coli cells.

The replacement of Escherichia coli recA gene (recA[Ec]) with the Pseudomonas aeruginosa recA(Pa) gene in Escherichia coli cells results in constitutive hyper-recombination (high frequency of recombination exchanges per unit length of DNA) in the absence of constitutive SOS response. To understand the biochemical basis of this unusual in vivo phenotype, we compared in vitro the recombination properties of RecA(Pa) protein with those of RecA(Ec) protein. Consistent with hyper-recombination activity, RecA(Pa) protein appeared to be more proficient both in joint molecule formation, producing extensive DNA networks in strand exchange reaction, and in competition with single-stranded DNA binding (SSB) protein for single-stranded DNA (ssDNA) binding sites. The RecA(Pa) protein showed in vitro a normal ability for cleavage of the E. coli LexA repressor (a basic step in SOS regulon derepression) both in the absence and in the presence (i.e. even under suboptimal conditions for RecA(Ec) protein) of SSB protein. However, unlike other hyper-recombinogenic proteins, such as RecA441 and RecA730, RecA(Pa) protein displaced insufficient SSB protein from ssDNA at low magnesium concentration to induce the SOS response constitutively. In searching for particular characteristics of RecA(Pa) in comparison with RecA(Ec), RecA441 and RecA803 proteins, RecA(Pa) showed unusually high abilities: to be resistant to the displacement by SSB protein from poly(dT); to stabilize a ternary complex RecA::ATP::ssDNA to high salt concentrations; and to be much more rapid in both the nucleation of double-stranded DNA (dsDNA) and the steady-state rate of dsDNA-dependent ATP hydrolysis at pH7.5. We hypothesized that the high affinity of RecA(Pa) protein for ssDNA, and especially dsDNA, is the factor that directs the ternary complex to bind secondary DNA to initiate additional acts of recombination instead of to bind LexA repressor to induce constitutive SOS response.