CLONING AND SEQUENCING OF THE SERRATIA-MARCESCENS GENE ENCODING A SINGLE-STRANDED DNA-BINDING PROTEIN (SSB) AND ITS PROMOTER REGION

The gene (ssb) coding for a single-stranded DNA-binding protein (SSB) was identified on a 1.2-kb EcoRI-SalI fragment cloned from chromosomal DNA of Serratia marcescens. The cloned fragment conferred increased resistance against UV and mitomycin C (MC) to ssb- mutants of Escherichia coli. The nucleotide (nt) sequence revealed that SSB consists of 175 amino acids (aa) and has an M(r) of 18 677. It shows 89% aa sequence homology with the SSB of E. coli. The nt sequence preceding the gene contains three promoters. Two of them overlap with a presumptive SOS box, and the distal one overlaps with a second SOS box that coincides with the promoter of the adjacent uvrA (gene encoding the UvrA protein). The uvrA is transcribed in a direction opposite to that of ssb. The sequence coding for the N terminus of the UvrA of S. marcescens indicates that the first 74 aa are identical to those of the E. coli protein. The results suggest that the two bacterial SSBs are members of a group which differs from the known SSBs of prokaryotic transmissible plasmids, because their aa sequence homology with these proteins is only about 60%.