The Function of E. coil Single-Stranded DNA Binding Protein and Expression of the ssb Gene

E. coli single-stranded DNA binding protein (SSB) has a role in DNA replication, repair, and recombination. The function of SSB in SOS induction and repair was examined using a gene fusion of recA to lacZ as an indicator of induction. The ssb-1 mutant, which is UV sensitive and temperature sensitive for replication, was found to be deficient in recA induction following treatments with UV light, mitomycin C and nalidixic acid. The ssb-113 mutant, which is UV sensitive, induced recA at almost normal levels. SSB thus is needed for both SOS induction and an additional repair process, which may be post-replication recombinational repair. Expression of the ssb gene was examined using several fusions of ssb to lacZ. The levels of ssb-lacZ expression remained fairly constant under all conditions tested. The largest fluctuation was a two-fold increase found after DNA damage and under special growth conditions (high cell density in L broth). Two regions of DNA sequence 5' to ssb were identified as affecting gene expression. One of these regions is the presumed ssb promoter; the other is 5' to this promoter and was found to cause higher basal levels and appeared to be responsible for the increase in expression following DNA damage.