Defective complex formation between single-stranded DNA and mutant recA430 or recA1 protein of Escherichia coli

The formation of a complex between recA protein and single-stranded DNA (ssDNA) was studied by filter-binding assays and sucrose density gradient centrifugation. In the presence of excess ATP, the wild-type recA protein formed salt-resistant complexes with ssDNA. One mutant recA430 (lexB30) protein bound to ssDNA slightly less effectively than wild-type protein, and the complexes had lost the stability to salt. Another mutant recA1 protein did not form complexes with ssDNA. On the other hand, in the absence of ATP, all proteins bound to ssDNA with the same efficiency, but all of the complexes were unstable in the presence of salt. The hybridization reaction in which homologous ssDNA is converted to double-stranded DNA (dsDNA) catalyzed by the recA430 protein was more sensitive to salt than that catalyzed by the wild-type protein. No hybridization reaction was found with the recA1 protein.