Abstract
Essential to the two distinct cellular events of genetic recombination and SOS induction in Escherichia coli, RecA protein promotes the homologous pairing and exchange of DNA strands and the proteolytic cleavage of the LexA repressor, respectively. Since both of these activities require single-stranded DNA (ssDNA) and ATP, the inter-relationship between these reactions was investigated and found to display many parallels. The extent of active complex formed between RecA protein and M13 ssDNA, as measured by both ATP hydrolysis and LexA proteolysis, is stimulated in a similar manner by either a reduction in magnesium ion concentration or the presence of single-stranded DNA binding (SSB) protein. However, unexpectedly, SSB protein inhibits both LexA proteolysis and ATP hydrolysis (in assays containing repressor) at concentrations of RecA protein that are substoichiometric to the ssDNA, arguing that LexA repressor affects the competition between RecA and SSB proteins for limited ssDNA binding sites. Additionally, attenuation of LexA repressor cleavage in the presence of double-stranded DNA or by an excess of ssDNA suggests that interaction of the RecA nucleoprotein filament with either LexA repressor or a secondary DNA molecule is mutually exclusive. The significance of these results is discussed in the context of both the regulation of inducible responses to DNA damage, and the competitive relationship between the processes of SOS induction and genetic recombination.
Highlights
Essential to the two distinct cellular events of genetic recombination and SOS induction in Escherichia coli, RecA protein promotes the homologous pairing and exchange of DNA strands and the proteolytic cleavage of the LexA repressor, respectively
Maximum rates of ATP hydrolysis are normally observed at a ratio of one RecA protein monomer per three nucleotides of single-stranded DNA (ssDNA)
Secondary structure intrinsic to ssDNA is stabilized by elevated levels of magnesium ion and prohibits RecA protein from saturating the ssDNA [31]; this results in apparently lower maximal rates of ATP hydrolysis and higher binding stoichiometries [30]
Summary
Essential to the two distinct cellular events of genetic recombination and SOS induction in Escherichia coli, RecA protein promotes the homologous pairing and exchange of DNA strands and the proteolytic cleavage of the LexA repressor, respectively. Since both of these activities require single-stranded DNA (ssDNA) and ATP, the inter-relationship between these reactions was investigated and found to display many parallels. Attenuation of LexA repressor cleavage in the presence of double-stranded DNA or by an excess of ssDNA suggests that interaction of the RecA nucleoprotein filament with either LexA repressor or a secondary DNA molecule is mutually exclusive The significance of these results is discussed in the context of both the regulation of inducible responses to DNA damage, and the competitive relationship between the processes of SOS induction and genetic recombination.
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