Functional domains of Escherichia coli single-stranded DNA binding protein as assessed by analyses of the deletion mutants

A series of C- and N-terminal deletion mutants of Escherichia coli single-stranded DNA binding protein (SSB) was constructed, purified, and characterized in terms of ability to self-multimerize and to bind to DNA. High-performance gel filtration chromatography revealed that the amino acids 89-105 play a key role in the maintenance of homotetramer for native SSB of 177 amino acids. Interestingly, all of the N-terminal deletion mutants studied here were eluted as octamers, indicating that the N-terminal 11 residues are involved in the prevention of the formation of octamers. The binding of SSB and its deletion mutant proteins to single-stranded d(T)(n) was examined by gel mobility shift assay and circular dichroism spectroscopy. C-terminal deletion mutant proteins, SSB1-135 and SSB1-115, maintained high affinity and may be wrapped by single-stranded DNA (ssDNA) in the same way as in the case of native SSB. In contrast, deletion of the C-terninal region (residues 89-115) or N-terminal region (residues 1-11) caused a dramatic decrease in the binding affinity. Furthermore, two different stoichiometries of SSB in the complexes with d(T)(64), but not with d(T)(32), were observed for native SSB, SSB1-135, SSB1-115, and SSB37-177, suggesting that the (SSB)(65) and (SSB)(35) binding modes, as previously demonstrated [Lohman, T. M., & Overman, L. B. (1985) J. Biol. Chem. 260, 3594-3603; Bujalowski, W., & Lohman, T. M. (1986) Biochemistry 25, 7799-7802], occurred at lower and higher SSB concentrations,respectively, A functional map for SSB molecule was presented and discussed.