Bacillus subtilis Bacteriophage SPP1-encoded Gene 34.1 Product is a Recombination-dependent DNA Replication Protein

SPP1-encoded replication and recombination proteins, involved in the early steps of the initiation of concatemeric DNA synthesis, have been analyzed. Dimeric G34.1P exonuclease degrades, with a 5′ to 3′ polarity and in a Mg2+-dependent reaction, preferentially linear double-stranded (ds) DNA rather than single-stranded (ss) DNA. Binding of the replisome organizer, G38P, to its cognate sites (oriDNA) halts the 5′ to 3′ exonucleolytic activity of G34.1P on dsDNA. The G35P recombinase increases the affinity of G34.1P for dsDNA, and stimulates G34.1P activity on dsDNA, but not on ssDNA. Then, filamented G35P promotes limited strand exchange with a homologous sequence. The ssDNA binding protein, G36P, protects ssDNA from the G34.1P exonuclease activity and stimulates G35P-catalyzed strand exchange. The data presented suggest a model for the role of G34.1P during initiation of σ replication: G38P bound to oriDNA might halt replication fork progression, and G35P, G34.1P and G36P in concert might lead to the re-establishment of a unidirectional recombination-dependent replication that accounts for the direction of DNA packaging.