Bacterial single-stranded (ss) DNA-binding proteins (SSBs) play essential protective and organizational roles in genome biology. In their protective functions, SSBs bind and sequester ssDNA intermediates that are formed during genome maintenance reactions. As organizational centers, SSB/ssDNA complexes form dynamic protein-docking “hubs” at which over a dozen different DNA replication, recombination, and repair enzymes gain access to genomic substrates through direct interactions with SSB. This clustering of enzymes is thought to help integrate cellular genome maintenance reactions by facilitating the exchange of ssDNA substrates between DNA replication, recombination, and repair pathways. In all cases examined to date, the last ~6 residues of SSB’s flexible C-terminus (SSB-Ct) form its protein docking site. Eukaryotic SSBs also interact with a diverse array of genome maintenance proteins but, since they lack the SSB-Ct element found in bacterial SSBs, they do so through distinct mechanisms.
We have mapped the SSB-Ct binding sites in several genome maintenance proteins. The examples we have focused on include bacterial proteins involved in DNA replication (chi subunit from the replicative DNA polymerase), recombination (RecQ DNA helicase), and repair (Exonuclease I and Uracil DNA glycosylase). The SSB-docking sites on these proteins are remarkably similar in spite their lack of structural similarity. Binding to SSB often stimulates enzyme activity through recruitment of the protein partner to its SSB/DNA substrate. In cells, these interactions are essential for the proper localization and functions of SSB-associated proteins.
Future work aims to establish the full extent of the SSB interaction network though protein interaction studies. We plan to examine how multiple SSB-binding proteins might function cooperatively on SSB/ssDNA substrates and to determine the mechanisms that regulate assembly of SSB-interacting proteins on SSB/ssDNA complexes in cells. The essential nature of bacterial SSB/protein interactions also makes the complexes attractive targets for the development of novel antibacterial therapeutics that block SSB/protein interactions.