Regulation of ssb Gene Expression 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 SSB protein is encoded by an gene. Although the gene promoters harbor an SOS box, multiple studies over several decades failed to elucidate whether gene expression is inducible and SOS dependent. The SOS regulon is comprised of about 50 genes, whose transcription is coordinately induced under stress conditions. Using quantitative real-time PCR, we determined the gene expression kinetics in UV- and γ-irradiated and revealed that gene expression is elevated in irradiated cells in an SOS-dependent manner. Additionally, the expression of the gene was determined to indicate the extent of SOS induction. In a mutant with a constitutively induced SOS regulon, the gene was overexpressed in the absence of DNA damage. Furthermore, we measured gene expression by droplet digital PCR during unaffected bacterial growth and revealed that gene expression was equal in wild-type and SOS bacteria, whereas expression was higher in the former. This study thus reveals a complex pattern of gene expression, which under stress conditions depends on the SOS regulon, whereas during normal bacterial growth it is unlinked to SOS induction. The gene is SOS regulated in such a way that its basal expression is relatively high and can be increased only through stronger SOS induction. The remarkable SOS induction observed in undisturbed wild-type cells may challenge our notion of the physiological role of the SOS response in bacteria.