DNA replication of phi29 and related phages takes place via a strand displacement mechanism, a process that generates large amounts of single-stranded DNA (ssDNA). Consequently, phage-encoded ssDNA-binding proteins (SSBs) are essential proteins during phage phi29-like DNA replication. Single-stranded DNA-binding proteins (SSBs) destabilize double-stranded DNA (dsDNA) and bind without sequence specificity, but selectively and cooperatively, to single-stranded DNA (ssDNA) conferring a regular structure to it, which is recognized and exploited by a variety of enzymes involved in DNA replication, repair and recombination. Phage phi29 protein p5 is the SSB protein active during phi29 DNA replication. It protects ssDNA against nuclease degradation and greatly stimulates dNTP incorporation during phi29 DNA replication process. Binding of the SSB to ssDNA prevents non-productive binding of the viral DNA polymerase to ssDNA, and allows the release DNA polymerase molecules that are already titrated by the ssDNA. This effect would be of particular importance in phi29-like DNA replication systems, where large amounts of ssDNA are generated and SSB binding to ssDNA could favor efficient re-usage of templates [1]. This domain family is found in SSB proteins in phage phi-29, homologues are found in Pfam:PF00436.

1: Differential functional behavior of viral phi29, Nf and GA-1 SSB proteins. Gascon I, Lazaro JM, Salas M; Nucleic Acids Res. 2000;28:2034-2042. PMID:10773070