A single amino acid substitution (Y78R) at the dimerdimer interface of homotetrameric single stranded DNA binding protein from E.coli (EcoSSB) renders the protein a stable dimer. This dimer can bind singlestranded DNA albeit with greatly reduced affinity. In vivo this dimeric SSB cannot replace homotetrameric EcoSSB. Amino acid changes at the rim of the dimerdimer interface nearby (Q76K, Q76E) show an electrostatic interaction between a charged amino acid at position 76 and bound nucleic acid. In conclusion, nucleic acid binding to homotetrameric SSB must take place across both dimers to achieve functionally correct binding.
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