DNA polymerase actively and sequentially displaces single-stranded DNA-binding proteins

Single-stranded DNA-binding proteins (SSBs) protect transiently exposed ssDNA, yet how DNA polymerase (DNAp) displaces them during replication remains unclear. Using single-molecule force spectroscopy, dual-color imaging, and molecular dynamics simulations on bacteriophage T7 DNAp and SSB, we investigated molecular mechanisms underlying SSB displacement. T7 SSB modulates replication in a force-dependent manner: enhancing it at low tension by preventing secondary structures while impeding it at high tension. Dual-color imaging shows SSBs remain stationary as DNAp advances, supporting a sequential displacement model. Molecular dynamics suggests that DNAp actively lowers the SSB dissociation energy barrier through interactions mediated by the SSB C-terminal tail. FRET confirms close protein proximity during encounters. Optimal replication requires SSB saturation of ssDNA, establishing a delicate balance between protection and efficiency. This spatiotemporal coordination between DNAp and SSB is critical for resolving molecular collisions and may represent a general mechanism for resolving molecular collisions, ensuring both processivity and genomic integrity.