RecA protein — promoted λ repressor cleavage: complementation between RecA441 and RecA430 proteins in vitro

Abstract

Induction of prophage λ occurs in recA441 mutant lysogens after a shift to 42° C in the presence of adenine. If the synthesis of RecA441 protein is maintained at a low basal level by the presence of a second mutation in the recA441 gene, recA453, induction of prophage λ is prevented. The ability to induce prophage λ is restored by the introduction, on a transducing phage, of a second recA gene carrying the recA430 mutation; by itself, the RecA430 protein is devoid of activity against the λ repressor (Rebollo et al. 1984). In order to explain how the RecA430 protein might complement the RecA441 protein to provide λ repressor cleavage in a recA453-441 (recA430) diploid lysogen, we characterized the cleavage reaction catalysed by a mixture of these proteins in vitro. Our results suggest that, in the presence of dATP, the RecA441 and RecA430 proteins form mixed multimers on single-stranded DNA, in which the RecA441 protein molecules enhance the DNA binding affinity of RecA430 protein molecules, but RecA430 protein molecules support no cleavage of the λ repressor. Although the effects of the RecA430 and single-strand binding (SSB) proteins are similar in vitro, we show that the SSB protein cannot substitute for the RecA430 protein in restoring λ repressor cleavage in a recA453-441 lysogen. Comparison of the stimulatory effect of long single-stranded DNA with that of (dA)₁₄ oligonucleotides on the RecA441 protein-directed cleavage of the λ repressor in the presence of various nucleoside triphosphates (NTPs) indicates that the cooperative binding of the RecA441 protein to single-stranded DNA stabilizes the RecA protein-DNA complexes so that they remain intact long enough to support cleavage of the λ repressor. We conclude that the low basal level of the RecA441 protein in a recA453-441 cell is sufficient to cleave the λ repressor, under conditions where a normal basal level of RecA430 protein is also present allowing the formation of mixed multimers on single-stranded DNA regions normally present in the cell.