Any SPP1 DNA restriction fragment cloned into Bacillus subtilis plasmid pC194 or pUB110 increased the transduction frequency of the plasmid by SPP1 100to 1,000-fold over the transduction level of the plasmid alone. This increment was observed irrespective of whether a fragment contained the SPP1 packaging origin * Corresponding author.

The DNA of Bacillus subtilis bacteriophage SPP1 is terminally redundant and partially circularly permuted. To explain these parameters, we followed the Streisinger-Botstein models of phage maturation and assumed that packaging of SPP1 DNA begins at a unique genomic site (“ pac ”) and proceeds sequentially from there. We describe the sequence of about 1,000 nucleotides surrounding pac . This together with size determinations of small, pac -terminated restriction fragments has revealed heterogeneity of the natural pac ends of SPP1 DNA. Such ends fell in each DNA strand into a region of five to seven nucleotides. However, within this range more than 50% of all molecules terminated with defined cytosines on both strands, generating a 3′ protruding terminus. The nucleotide sequence of the DNA segment surrounding pac did not reveal any features which would distinguish this region.