THE SINGLE-STRANDED-DNA-BINDING PROTEINS (SSB) OF PROTEUS-MIRABILIS AND SERRATIA-MARCESCENS

The single-stranded-DNA-binding (SSB) proteins from Proteus mirabilis and Serratia marcescens were purified from overproducing Escherichia c oli strains, which were devoid of their own ssb gene. The strains harb oured an endA insertion mutation and a xonA mutation resulting in the absence of endonuclease I and exonuclease I activities from the prepar ations. The amino acid sequences of the SSB of all three species are n early identical in the N-terminal parts of the proteins that contain t he DNA-binding domain, but differ in the C-terminal parts. Both protei ns have an apparent binding-site size of 65 and 35 nucleotides at high and low salt concentrations, respectively. The association-rate const ant for binding to poly(dT) is 3.2X10(8)M(-1)s(-1) for P. mirabilis SS B (PmiSSB) and 3.4X10(8)M(-1)s(-1) for S. marcescens SSB (SmaSSB). The se binding parameters are very similar to those of E. coli SSB (EcoSSB ). The structural similarity of the proteins is also documented by the finding that they can exchange subunits among each other to form mixe d tetramers. The transcriptional regulation of the ssb and uvrA genes from P. mirabilis and S. marcescens in SOS-induced E. coli cells was s tudied using lacZ fusions. While the uvrA genes were inducible, there was no induction of the ssb genes transcribed divergently from the uvr A genes. Apparently, regions with nucleotide sequence similarity to th e E. coli SOS-box preceding the ssb genes of P. mirabilis and S. marce scens had no gross effect on the transcription. Studies on growth of t he cells and recovery from ultraviolet damage indicate that the hetero logous SSB proteins support DNA replication and recombinational DNA re pair of E. coli with the same efficiency as the E. coli SSB protein. I nteractions with other E. coli proteins involved in these processes ei ther do not occur, or are not impeded.