A possible interaction of single-strand binding protein and RecA protein during post-ultraviolet DNA synthesis

The mechanism of DNA replication in ultraviolet (UV)-irradiated Escherichia coli is proposed. Immediately after UV exposure, the replisome aided by single-strand DNA-binding protein (SSB) can proceed past UV-induced pyrimidine dimers without insertion of nucleotides. Polymerisation eventually resumes somewhere downstream of the dimer sites. Due to the limited supply of SSB, only a few dimers can be bypassed in this way. Nevertheless, this early DNA synthesis is of great biological importance because it generates single-stranded DNA regions. Single-stranded DNA can bind and activate RecA protein, thus leading to induction of the SOS response. During the SOS response, the cellular level of RecA protein increases dramatically. Due to the simultaneous increase in the concentration of ATP, RecA protein achieves the high-affinity state for single-stranded DNA. Therefore it is able to displace DNA-bound SSB. The cycling of SSB on and off DNA enables the replisome to bypass a large number of dimers at late post-UV times. During this late replication, the stoichiometric amounts of RecA protein needed for recombination are involved in the process of postreplication repair.