ssb gene expression is SOS-dependent in Escherichia coli

Bacterial SSB proteins, as well as their eukaryotic RPA analogues, are essential and ubiquitous. They avidly bind single-stranded DNA and regulate/coordinate its metabolism, hence enabling essential DNA processes such as replication, transcription, and repair. The prototypic Escherichia coli SSB protein is encoded by an ssb gene. The ssb contains three promoters and a single LexA repressor binding sequence (SOS box), which implicates that ssb belongs to the SOS regulon. The SOS regulon is a set of about 50 genes in E. coli chromosome whose expression is repressed by a LexA repressor, until coordinately activated in stress conditions. Since SSB is such an important governor of DNA metabolism, it should be unsurprising that it is a part of the regulon that regulates DNA replication, repair, mutagenesis, and cell division in bacteria under stress. However, the issue of SOS regulation of E. coli ssb expression turns out to be far from settled. Namely, LexA may repress only one ssb promoter, whereas the other two enable constitutive, though low, ssb expression. The LexA binding box is rather distant (-170 nt) from the ssb coding region and is divergent from the consensus SOS box. Whether the E. coli ssb is inducible under SOS response was explored by several laboratories in the 1980s and then again in the 2000s. None of the more than 10 studies showed SOS-dependence of ssb expression. We determined a time-course of ssb and sulA (as an indication of SOS induction) gene expression by using sensitive and reliable methods, such as quantitative real-time PCR (qPCR) and droplet digital PCR (ddPCR).