Probing E. coli SSB protein-DNA topology by reversing DNA backbone polarity

Escherichia coli single-strand (ss) DNA binding protein (SSB) is an essential protein that binds ssDNA intermediates formed during genome maintenance. SSB homotetramers bind ssDNA in two major modes, differing in occluded site size and cooperativity. The (SSB)₃₅ mode in which ssDNA wraps, on average, around two subunits is favored at low [NaCl] and high SSB/DNA ratios and displays high unlimited, nearest-neighbor cooperativity forming long protein clusters. The (SSB)₆₅ mode, in which ssDNA wraps completely around four subunits of the tetramer, is favored at higher [NaCl] (>200 mM) and displays limited low cooperativity. Crystal structures of E. coli SSB and Plasmodium falciparum SSB show ssDNA bound to the SSB subunits (OB folds) with opposite polarities of the sugar phosphate backbones. To investigate whether SSB subunits show a polarity preference for binding ssDNA, we examined EcSSB and PfSSB binding to a series of (dT)₇₀ constructs in which the backbone polarity was switched in the middle of the DNA by incorporating a reverse-polarity (RP) phosphodiester linkage, either 3′-3′ or 5′-5′. We find only minor effects on the DNA binding properties for these RP constructs, although (dT)₇₀ with a 3′-3′ polarity switch shows decreased affinity for EcSSB in the (SSB)₆₅ mode and lower cooperativity in the (SSB)₃₅ mode. However, (dT)₇₀ in which every phosphodiester linkage is reversed does not form a completely wrapped (SSB)₆₅ mode but, rather, binds EcSSB in the (SSB)₃₅ mode with little cooperativity. In contrast, PfSSB, which binds ssDNA only in an (SSB)₆₅ mode and with opposite backbone polarity and different topology, shows little effect of backbone polarity on its DNA binding properties. We present structural models suggesting that strict backbone polarity can be maintained for ssDNA binding to the individual OB folds if there is a change in ssDNA wrapping topology of the RP ssDNA.